THE SKELETON OF THE FROG[55] (_Rana temporaria_).
I. EXOSKELETON.
The skin of the frog is smooth and quite devoid of scales or other
exoskeletal structures. The only exoskeletal structures met with in
the frog are:--
1. The =teeth=, which are most conveniently described with the
endoskeleton.
2. The horny covering of the calcar or prehallux (see p. 167).
II. ENDOSKELETON.
The endoskeleton of the adult frog consists partly of cartilage,
partly of bone and each of these types of tissue occurs in two forms.
The cartilage may be hyaline, as in the omosternum and xiphisternum,
or may be more or less calcified as in part of the suprascapula and
the epiphyses of the limb bones. The bone may be cartilage bone, or
membrane bone.
The skeleton is divisible into an =axial portion= consisting of the
skull, vertebral column, and sternum, and an =appendicular portion=
consisting of the skeleton of the limbs and their girdles.
1. THE AXIAL SKELETON.
A. THE VERTEBRAL COLUMN.
The vertebral column is a tube, formed of a series of ten bones which
surround and protect the spinal cord. Of these ten bones nine are
vertebrae, while the tenth is a straight rod, the =urostyle=, and is
almost as long as all the vertebrae put together. The second to eighth
vertebrae inclusive have a very similar structure, but the first and
ninth differ from the others.
Any one of the second to eighth vertebrae forms a bony ring with a
somewhat thickened floor, the =centrum= or body, which articulates
with the centra of the immediately preceding and succeeding vertebrae.
The articulating surfaces are covered with cartilage and are
procoelous, or convex in front and concave behind. The eighth vertebra
is however amphicoelous or biconcave. The centrum of each vertebra
encloses an isolated vestige of the notochord. The =neural arch= forms
the roof and sides of the neural canal, which is very spacious in the
anterior vertebrae, but becomes more depressed in the posterior ones.
The arch bears the =neural spine=, a low median ridge of variable
character, and is drawn out in front and behind, forming the two pairs
of articulating surfaces or =zygapophyses= by means of which the
vertebrae are attached together. Of these the anterior articulating
surfaces or =prezygapophyses= look upwards and slightly inwards, while
the posterior articulating surfaces or =postzygapophyses= look
downwards and slightly outwards. The sides of the neural arches are
drawn out into a pair of prominent =transverse processes=. Those of
the second vertebra look somewhat forwards, those of the third look
directly outwards or somewhat forwards, while those of the fourth,
fifth, and sixth are directed slightly backwards, and those of the
seventh and eighth nearly straight outwards. All the transverse
processes are terminated by very small cartilaginous =ribs=.
SPECIAL VERTEBRAE.
The =first vertebra= is a ring-like structure with a much depressed
centrum. It bears in front two oval concave surfaces for articulation
with the condyles of the skull, while the centrum is terminated behind
by a prominent convex surface. There are as a rule no transverse
processes, and the postzygapophyses look downwards and outwards.
Occasionally however transverse processes do occur. Projecting
forwards from the centrum is a minute process better developed in the
Newt. This resembles an odontoid process, and it has hence been
supposed that the first vertebra is homologous with the axis of
mammalia, and that the atlas of the frog is fused with the skull.
The =ninth vertebra= has very stout transverse processes directed
backwards and somewhat upwards. They articulate with the pelvic girdle
and hence this vertebra is regarded as the =sacrum=. The neural arch
is much depressed, the centrum is convex in front and bears on its
posterior surface two short rounded processes for articulation with
the urostyle.
The =urostyle= is a long rod-like bone forming the posterior
unsegmented continuation of the vertebral column. It is probably
equivalent to three vertebrae, the tenth, eleventh, and twelfth fused
together, and to an unsegmented rod of cartilage which lies ventral to
the notochord. The anterior end is expanded and bears two concave
articular surfaces by means of which it articulates with the sacrum. A
prominent ridge runs along the dorsal surface, but gradually
diminishes when traced back. The anterior portion contains a canal
which is a continuation of the neural canal. At a point not far from
the anterior end, this canal communicates with the exterior by a pair
of minute holes which correspond with the intervertebral foramina.
B. THE SKULL[56].
The skull of the Frog consists of three principal parts:--
(1) an axial part, the =cranium proper=, which encloses the brain. To
it are firmly fused
(2) the =capsules of the olfactory and auditory sense organs=,
(3) lastly there is the =hyoid apparatus= and the =skeleton of the
jaws=.
The skull is by no means so completely ossified as is the vertebral
column, but in addition to the cartilage bone, there is a great
development of membrane bone in connection with it.
The skull has a peculiarly flattened and expanded form depending on
the wide lateral separation of the jaws from the cranium.
(1) THE CRANIUM PROPER or Brain case.
This is an unsegmented tube, which is widest behind. It remains to a
considerable extent cartilaginous, but is partly converted into
cartilage bone, partly sheathed in membrane bone. Its roof is
imperfect, being pierced by three holes or =fontanelles=, one large
anterior fontanelle (fig. 25, A, 9), and two smaller posterior
fontanelles (fig. 25, A, 10).
The cartilage bones of the cranium proper are the two =exoccipitals=
and the =sphenethmoid=.
The =exoccipitals= (figs. 24, 25, and 26, 6) are a pair of irregular
bones bounding the foramen magnum at the posterior end of the skull.
They almost completely surround the foramen magnum, and bear a pair of
oval convex surfaces, the =occipital condyles=, with which the first
vertebra articulates. The bones generally called the exoccipitals of
the frog include the =epi-otic= and =opisthotic= elements of many
skulls, in addition to the exoccipitals.
[Illustration FIG. 24. A DORSAL, AND B VENTRAL VIEWS OF THE CRANIUM OF
A COMMON FROG (_Rana temporaria_) × 2 (after PARKER).
In this and the next two figs. cartilage is dotted, cartilage bones
are marked with dots and dashes, membrane bones are left white.
1. sphenethmoid.
2. fronto-parietal.
3. pterygoid.
4. squamosal.
6. exoccipital.
7. parasphenoid.
8. pro-otic.
9. quadratojugal.
10. maxillae.
11. nasal.
12. premaxillae.
13. anterior nares.
14. vomer.
15. posterior nares.
16. palatine.
18. columella.
19. quadrate.
20. occipital condyle.
II. optic foramen.
V. VII. foramen for exit of trigeminal and facial nerves.
IX. X. foramina for exit of glossopharyngeal and pneumogastric
nerves.]
The patch of unossified cartilage immediately external to the
occipital condyle is pierced by two small foramina, through which the
ninth and tenth nerves leave the cranial cavity. The ninth nerve
passes through the more external of these foramina, the tenth through
the one nearer the condyle. The foramina lie however very close
together and are sometimes confluent. The cranial walls for a
considerable distance in front of the occipitals are unossified, but
the anterior end of the cranial cavity is encircled by another
cartilage bone, the =sphenethmoid= (figs. 24 and 25, 1) or girdle
bone. This partly corresponds to the orbitosphenoids of the Newt's
skull. Anteriorly it is pierced by a pair of small foramina through
which the ophthalmic branches of the trigeminal nerve pass out.
The anterior part of the cranial cavity is divided into two halves by
a vertical plate, the =mesethmoid=. Some little distance behind the
sphenethmoid the ventro-lateral walls of the cartilaginous cranium are
pierced by a pair of rather prominent holes, the =optic foramina=
(figs. 24 and 25, B, II), and at a similar distance further back,
occupying a kind of notch in the pro-otic are the large =trigeminal
foramina=, through which the fifth and seventh nerves leave the
cranium. Between the trigeminal and optic foramina are the very small
foramina for the sixth nerves (fig. 25, B, VI).
The _membrane bones_ of the cranium proper include the
_fronto-parietals_ and the _parasphenoid_.
The _fronto-parietals_ (figs. 24 and 26, A, 2) form a pair of long
flat bones closely applied to one another in the middle line, the line
of junction being the =sagittal suture=. They cover over the
fontanelles and overlap the sphenethmoid in front.
The _parasphenoid_ (figs. 24 and 26, B, 7) is a bone shaped like a
dagger with a very short handle. It lies on the ventral surface of the
cranium, the blade being directed forwards and underlying the
sphenethmoid; its lateral processes underlie the auditory capsules.
(2) THE SENSE CAPSULES.
The sense capsules are cartilaginous or bony structures which surround
the olfactory and auditory organs and are closely united to the
cranium.
[Illustration FIG. 25. A DORSAL AND B VENTRAL VIEW OF THE CRANIUM OF A
COMMON FROG (_Rana temporaria_) from which the membrane bones have
mostly been removed. × 2 (after PARKER).
1. sphenethmoid.
2. palatine.
3. pterygoid.
4. quadrate.
5. columella.
6. exoccipital.
7. ventral cartilaginous wall of cranium.
8. pro-otic.
9. anterior fontanelle.
10. right posterior fontanelle.
11. quadratojugal.
12. nasal capsule.
II. V. VI. IX. X. foramina for exit of cranial nerves.]
The =auditory capsules= are fused with the sides of the posterior
end of the cranium just in front of the exoccipitals. They are
largely cartilaginous, but include in their anterior walls a pair
of irregular cartilage bones, the =pro-otics= (figs. 24 and 25, 8).
The cartilaginous area lying ventral to the pro-otic and external
to the exoccipital is pierced by a rather prominent hole, the
=fenestra ovalis=, which forms a communication between the internal
ear cavity, and a space the tympanic cavity, which lies at the side
of the head, and is bounded externally by the tympanic membrane. The
fenestra ovalis is occupied by a minute cartilaginous structure, the
=stapes=, and articulated partly to this and partly to a slight recess
in the pro-otic is the =columella= (fig. 25, B, 5), a rod in part
bony and in part cartilaginous, whose outer end is attached to the
tympanic membrane. The columella and stapes are together homologous
with the mammalian auditory ossicles and with the hyomandibular of
Elasmobranchs. Sometimes the term columella is used to include the
whole ossicular chain,--the columella together with the stapes.
The =olfactory= or =nasal capsules= (fig. 25, B, 12) are fused with
the anterior end of the cranium and differ from the auditory capsules
in being to a great extent unossified. There are however two pairs of
membrane bones developed in connection with them, the _vomers_ and the
_nasals_. They are drawn out into three pairs of cartilaginous
processes, on the dorsal surface into the =prenasal= and =alinasal=
processes which bound the external nares, and on the ventral surface
towards the middle line into the forwardly-projecting =rhinal=
processes.
The _nasals_ (figs. 24 and 26, 11) form a pair of triangular bones
lying dorsolaterally in front of the fronto-parietals. Their bases are
turned towards one another and their apices are directed outwards and
backwards. They correspond in position with the prefrontals of the
reptilian skull as well as with the nasals.
The _vomers_ are a pair of irregular bones lying on the ventral
surface of the olfactory capsules. Each bears on its inner and
posterior angle a group of minute pointed teeth, while its outer
border is drawn out into three or four small slightly diverging
processes, the two posterior of which form the inner boundary of the
=posterior nares= (fig. 24, B, 15).
(3) THE JAWS.
The =upper jaw= consists of a rod of cartilage connected with the
cranium near its two ends, but widely separated from it in the middle.
It is almost completely overlain by membrane bone. With its posterior
end the lower jaw articulates.
The membrane bones of the upper jaw include first the _premaxillae_, a
small bone meeting its fellow in the middle line, and forming the
extreme anterior end of the upper jaw. It gives off on its dorsal
surface a backwardly-projecting process. It is connected behind with
the _maxillae_ (figs. 24 and 26, 10), a long flattened bone which forms
the greater part of the margin of the upper jaw, and gives off near
its anterior end a short process which projects upwards and meets the
nasal.
[Illustration FIG. 26. A, LATERAL VIEW OF THE SKULL, B, POSTERIOR VIEW
OF THE CRANIUM OF A COMMON FROG (_Rana temporaria_) × 2 (after
PARKER).
1. sphenethmoid.
2. fronto-parietal.
3. pterygoid.
4. squamosal.
5. tympanic membrane.
6. exoccipital.
7. parasphenoid.
8. pro-otic.
9. quadratojugal.
10. maxillae.
11. nasal.
12. premaxillae.
13. anterior nares.
14. mento-meckelian.
15. dentary.
16. angulo-splenial.
17. basilingual plate.
19. quadrate.
20. columella.
21. occipital condyle.
22. anterior cornu of the hyoid (cerato-hyal).
23. foramen magnum.
II. IX. X. foramina for the exit of cranial nerves.]
Both maxillae and premaxillae are grooved ventrally, and bear, attached
to the outer more prominent margin of the groove, a row of minute
conical teeth. These teeth are =pleurodont=, that is, are ankylosed by
their bases and outer sides to the margin of the jaw. Each tooth is a
hollow cone, the basal part of which is formed of bone, the apical
part of dentine, capped by a very weak development of enamel.
The posterior end of the maxillae is overlapped by a small bone, the
_quadratojugal_ (figs. 24 and 26, 9), whose posterior end forms part
of the articular surface for the lower jaw. Just behind the
quadratojugal there is a small unossified area which lies at the angle
of the mouth, and is connected by a narrow bar of cartilage with the
cranium; this forms the =quadrate= (figs. 24 and 26, 19). A
backwardly-directed outgrowth from the cartilaginous bar more or less
completely surrounds the tympanic membrane, forming the tympanic ring.
When followed back the maxillae and quadratojugal diverge further and
further from the cranium, till the angle of the mouth comes to be
separated from the foramen magnum by a space nearly double the width
of the cranium. This space is bridged over to a considerable extent by
two triradiate bones, the _pterygoid_ and _squamosal_.
The _pterygoid_ (figs. 24 and 26, 3) is a large bone, whose anterior
limb runs forwards meeting the maxillae and palatine; while its inner
limb meets the auditory capsule and parasphenoid, and its outer limb
runs backwards and outwards to the angle of the mouth. The _palatine_
is a small transversely-placed bone, which connects the pterygoid with
the anterior part of the sphenethmoid. The _squamosal_ (figs. 24 and
26, 4) is a T-shaped bone whose anterior arm is pointed and passes
forwards to meet the pterygoid. The posterior upper arm is closely
applied to the pro-otic, while the posterior lower arm meets the
pterygoid and quadratojugal at the angle of the jaw, and surrounds the
narrow cartilaginous bar of the quadrate which goes to join the
cranium. The squamosal is probably homologous with the squamosal
together with the pre-opercular of Bony Ganoids.
The quadrate and squamosal form the =suspensorium= by which the lower
jaw is connected with the cranium.
The =lower jaw= or =mandible= consists of a pair of cartilaginous rods
(=Meckel's cartilages=) in connection with each of which there are
developed two membrane bones and one cartilage bone. The cartilage
bone is the =mento-meckelian= (fig. 26, A, 14), a very small
ossification at the extreme anterior end. The membrane bones are the
_angulo-splenial_ and the _dentary_. The _angulo-splenial_ is a strong
flat bone which forms the inner and lower part of the mandible for the
greater part of its length. Its dorsal surface is produced into a
slight =coronoid process=. The _dentary_ (fig. 26, A, 15) is a flat
plate which covers the outer surface of the anterior half of the
mandible, as far forwards as the mento-meckelian. The lower jaw is
devoid of teeth. The part of Meckel's cartilage which in most
vertebrates ossifies, forming the articular bone, remains unossified
in the Frog.
THE HYOID APPARATUS.
The =hyoid= of the adult Frog is formed of the modified hyoid and
branchial arches of the tadpole. It consists of a broad thin plate of
cartilage, the =basilingual plate= (fig. 29, B, 1), drawn out into two
pairs of long processes, the =cornua=. The basilingual plate is
broader in front than behind, and is formed from the fused ventral
ends of the hyoid and branchial arches of the tadpole.
The =anterior cornua= (fig. 29, B, 2) form a pair of long slender
cartilaginous rods which project from the body of the hyoid at first
forwards, then backwards, and finally upwards and somewhat forwards
again, to be united to the auditory capsules just below the fenestrae
ovales. They are formed from the dorsal portion of the hyoid arch of
the tadpole and are homologous with the cerato-hyals of the Dogfish.
The =posterior cornua= form a pair of straight bony rods diverging
outwards from the posterior end of the basilingual plate. They are
formed from the fourth branchial arches of the tadpole, and differ
from the rest of the hyoid apparatus in being well ossified.
The =columellar chain=, which has been already described (p. 157),
should be mentioned with the hyoid as it is homologous to the
hyomandibular of fishes.
The =sternum= of the =Frog=, though regarded as part of the axial
skeleton, is so intimately connected with the pectoral girdle, that it
will be described with the appendicular skeleton.
2. THE APPENDICULAR SKELETON.
This consists of the skeleton of the two pairs of limbs and their
respective girdles. It is at first entirely cartilaginous but the
cartilage becomes later on mainly replaced by bone. The only bone
developed in connection with the appendicular skeleton, which has no
cartilaginous predecessor, is the _clavicle_.
A. THE PECTORAL GIRDLE.
This consists originally of two half rings of cartilage encircling the
sides of the body a short way behind the head. These two halves meet
one another in the ventral middle line, and separate the anterior
elements of the sternum from the posterior ones.
Each half-ring bears on the middle of its outer and posterior surface
a prominent cup, the =glenoid cavity=, with which the proximal
arm-bone articulates. This cup divides the half-arch into a dorsal
=scapular= and a ventral =coracoid= portion.
The =scapular portion= consists of two parts, the =suprascapula= and
the =scapula=.
The =suprascapula= (fig. 30, A, 2) is a wide, thin plate attached by
its ventral and narrowest border to the scapula. Its proximal and
anterior half is imperfectly ossified, its whole border or sometimes
only its dorsal and posterior borders consist of unaltered hyaline
cartilage, while the rest of it is composed of calcified cartilage.
The =scapula= (fig. 30, A, 3) is a fairly stout rod of bone
constricted in the middle, and forming the dorsal half of the glenoid
cavity.
The =coracoid portion= consists of three parts, the =coracoid=,
=precoracoid= and _clavicle_.
The largest and most posterior of these is the _coracoid_ (fig. 30, A,
4) which like the scapula, is contracted in the middle and expanded at
the ends, especially at the ventral end. It forms a large part of the
glenoid cavity. The ventral ends of the coracoids which meet one
another in the middle line are unossified, and form narrow strips of
calcified cartilage, the =epicoracoids= (fig. 30, A, 5); these are
often regarded as sternal elements.
The =precoracoid= forms a narrow strip of cartilage lying in front of
the coracoid, from which it is separated by the wide =coracoid
foramen= (fig. 30, A, 9). The dorsal end is continuous with an area of
unossified cartilage which separates the coracoid and scapula and
forms part of the glenoid cavity.
The _clavicle_ is a narrow membrane bone closely attached to the
anterior surface of the precoracoid, its dorsal end is expanded.
THE STERNUM.
The sternum consists of four parts arranged in two groups; two parts
to each group. The anterior members are the episternum and omosternum.
The =episternum= (fig. 30, A, 10) is a thin almost circular plate of
cartilage much of which remains hyaline.
The =omosternum= (fig. 30, A, 11) is a slender bony rod widest at its
posterior end; it connects the episternum with the ventral ends of the
precoracoids.
The =sternum proper= is a short rod of cartilage sheathed in bone; it
is contracted in the middle and expanded at each end. It bears
attached to its posterior end a broad somewhat bilobed plate of
partially calcified cartilage, the =xiphisternum= (fig. 30, A, 13).
B. THE ANTERIOR LIMB.
This is divisible into three parts, the =upper arm= or =brachium=, the
=fore-arm= or =antibrachium=, and the =manus=.
All the larger bones have their ends formed by prominent epiphyses
which do not unite with the shaft till late in life. Their
articulating surfaces are covered by hyaline cartilage.
In the =upper arm= there is a single bone, the =humerus=.
This has a more or less cylindrical shaft and articulates by a
prominent rounded =head= with the glenoid cavity. The distal end shows
a large rounded swelling on either side of which is a =condylar
ridge=, the inner or postaxial one being the larger. A prominent
=deltoid ridge= runs along the proximal half of the anterior surface,
and in the male frog a second equally prominent ridge runs along the
distal half of the posterior surface.
The =fore-arm= consists of two bones, the =radius= and =ulna=, united
together and forming the =radio-ulna=. The two bones are quite fused
at their proximal ends where they form a deep cup which articulates
with the distal end of the humerus, and is drawn out into a rather
prominent backwardly-projecting =olecranon process=, which ossifies
from a centre distinct from that of the shaft. The distal end is
distinctly divided by a groove into an anterior radial and a posterior
ulnar portion.
The =manus= consists of two parts, the =wrist= or =carpus= and the
=hand=.
The =carpus=[57] consists of six small bones arranged in two rows. The
three bones of the proximal row are the =ulnare=, =radiale= and
=centrale=. The =ulnare= and =radiale= are about equal in size and
articulate regularly with the radio-ulna. The =centrale= is pushed out
of its normal position and lies partly on the pre-axial side, partly
in front of the radiale. Of the three bones of the distal row the two
pre-axial ones, =carpalia 1= and =2=, are small; carpale 2 articulates
with the second metacarpal, carpale 1 with both the first and second.
The third bone is large and articulates with the third, fourth and
fifth metacarpals, it represents =carpalia 3-5=, with probably in
addition the representative of a second centrale.
The =hand= consists of four complete digits, and a vestigial =pollex=
reduced to a short metacarpal.
Each of the four complete digits consists of a =metacarpal= and a
variable number of =phalanges=. The first digit, as just mentioned,
has no phalanges, the second and third have two, and the fourth and
fifth have three.
C. THE PELVIC GIRDLE.
The pelvic girdle of the Frog is much modified from the simple or
general type found in the Newt (p. 149).
It is a V-shaped structure consisting of two halves which are fused
together in the middle line posteriorly, while in front they are
attached to the ends of the transverse processes of the sacral
vertebra. Each half bears at its posterior end a deep cup, the
=acetabulum=, with which the head of the femur articulates.
Each half of the pelvis ossifies from two centres. The anterior and
upper half of the acetabulum, and the long laterally compressed bar
extending forwards to meet the sacral vertebra ossify from a single
centre and are generally called the =ilium=; it is probable however
that they represent both the =ilium= and =pubis= of mammals[58]. The
posterior part of this bone meets its fellow in a median symphysis.
The posterior third of the acetabulum is formed by a small bone, the
=ischium=, which likewise meets its fellow in a median symphysis.
The ventral portion of the pelvic girdle never ossifies, even in old
animals being formed only of calcified cartilage. This is generally
regarded as the pubis, but it perhaps corresponds to the =acetabular
bone= of mammals.
D. THE POSTERIOR LIMB.
This corresponds closely to the anterior limb and, like it, is
divisible into three parts, the =thigh=, the =shin= or =crus= and the
=pes=.
As was the case with the anterior limb, all the long bones have their
ends formed by prominent epiphyses which do not unite with the shaft
till late in life.
In the =thigh= there is only a single bone, the =femur=.
The =femur= is a moderately long, slender bone with a well-ossified
hollow shaft slightly curved in a sigmoid manner. Both ends are
expanded, the proximal end is hemispherical and articulates with the
acetabulum, the distal end is larger and more laterally expanded.
The =shin= likewise includes a single bone, the =tibio-fibula=, but
this, as can be readily seen by the grooves at the proximal and distal
ends of the shaft, is formed by the fusion of two distinct bones, the
=tibia= and =fibula=. The tibio-fibula is longer and straighter than
the femur.
The =pes= consists of two parts, the =ankle= or =tarsus= and the
=foot=.
The =tarsus= consists of two rows of structures, very different in
size. The proximal row consists of two long bones, the =tibiale= and
=fibulare=, which are united by common epiphyses at the two ends,
while in the middle they are widely separated. The tibiale lies on the
tibial or pre-axial side, and the fibulare which is the larger of the
two bones on the fibular or postaxial side. The distal row of tarsals
consists of three very small pieces of calcified cartilage. The
postaxial of these is the largest, it articulates with the second and
third metatarsals and is probably homologous with tarsalia 2 and 3
fused. The middle one is very small, it articulates with the first
metatarsal and is probably tarsale 1. The pre-axial one articulates
with the metatarsal of the calcar, a structure to be described
immediately, and has been regarded as a =centrale=.
The =foot= includes five complete digits and a supplemental toe as
well. Each of the five digits consists of a long =metatarsal= with
epiphyses at both ends, and of a variable number of phalanges. The
first digit or =hallux= and the second have two phalanges, the third
three, the fourth, which is the largest, four, and the fifth, three.
The distal phalanges have epiphyses only at their proximal ends, the
others at both ends.
On the pre-axial side of the hallux is the supplemental digit, the
=prehallux= or =calcar=. It consists of a short metatarsal and one or
two phalanges, and is terminated distally by a horny covering of
epidermal origin.
FOOTNOTES:
[55] See A. Ecker, _Die anatomie des Frosches_, Braunschweig 1864,
translated by G. Haslam, Oxford, 1889, also A.M. Marshall, _The Frog_,
5th edition, Manchester and London, 1894.
[56] W.K. Parker, _Phil. Trans._ 161, 1871, p. 137, and W.K. Parker
and G.T. Bettany, _The Morphology of the Skull_, London, 1877, p. 136.
[57] See G.B. Howes and W. Ridewood, _P.Z.S._, 1888, p. 141.
[58] See bottom of p. 187.