This fine genus of trilobites was established November, 1824, by my
friend James E. Dekay, M. D. It embraces a considerable number of
species so analogous to each other, that except in a very few cases,
it is exceedingly difficult to point out their distinctive characters.
Some of the species of Isotelus, appear to have reached a greater size
than any other trilobite. In the cabinet of P. A. Browne, Esq., there
is the fragment of one, which must have been fourteen or fifteen inches
long.
The Isotelus is found in several parts of North America, but most
abundantly in the black transition limestone, in the northern section
of the country. The richest locality, not only of this genus, but also
of the Calymene and the Asaph, is Trenton Falls, on West Canada Creek,
about 13 miles to the north of Utica, in the state of New York. The
following extracts from the notes of Professor Renwick, which accompany
Dr. Dekay's account of the Isotelus, will give some idea of this vast
depository of the medals of ancient zoology. West Canada Creek, is one
of the principal branches of the Mohawk River. At Trenton Falls it has
worn itself a passage through the rock for the distance of nearly two
miles, forming a series of water falls; and has thus laid open to view
the strata to the depth of probably 300 feet. The layers of the rock
thus disclosed are nearly horizontal, and of various thicknesses: they
are composed of limestone, with the exception of numerous thin veins
of argillaceous matter. The higher strata are composed of carbonate of
lime nearly pure, of a light grey colour and crystalline structure. At
greater depths it is more compact and darker in colour, and finally it
appears quite black and highly fœtid.[36]
[Footnote 36: See Annals of N. Y. Lyceum, vol. i. page 185.]
Animal remains are contained in every part of the rock; besides several
genera of trilobites, we have several species of orthocera. Encrinites
and Fungites--Nautili--Terebratulæ and Producti, are quite common.
The favosites here are sometimes six inches in diameter, and in such
numerous columns, as to have induced the late worthy proprietor of
this interesting spot, Mr. J. Sherman, to consider them as analogous
in structure to the basaltic columns of Staffa and the Giant's
Causeway; he therefore maintains the extravagant theory that these
columns are nothing more than gigantic favosites.[37] We visited this
famous locality of trilobites not long since, and were almost as much
delighted with the sublimity and grandeur of the cataract, and the
picturesque and romantic character of the glen, as with the reliques of
olden times, which are scattered here in such profusion.
[Footnote 37: See a Description of Trenton Falls, by John Sherman, p.
17.]
The genus Isotelus, derived from Ἱσος, equal, and τελος, extremity, is
thus characterized by Dr. Dekay.
_Body_ oval often contracted, not unfrequently extended.
_Head_ or buckler large and rounded, equalling the tail in size, but
with two oculiform tubercles.
_Abdomen_ with eight articulations.
Frontal process beneath, with two semilunar terminations.
_Post abdomen_ or tail, broad, expanded with indistinct divisions, as
large as the buckler.
Longitudinal lobes very distinct.
Other distinguishing marks by which this genus may be known, have been
given in our introduction.
Isotelus Gigas. _Dekay_. Casts Nos. 21 and 22.
_Head_ representing a spherical triangle, surface punctate, convex,
descending from between the eyes to the anterior border, which has
a narrow raised rim; posterior extremity concave and corresponding
to the articulation of the abdomen. Eyes elevated, prominent,
sub-pedunculated; cornea oblong, lunated, highly polished; _abdomen_
with eight distinct articulations, the middle lobe double the size
of the lateral one: these latter are continuous with the middle
lobe, have a deep furrow impressed on their upper surfaces, which
becomes gradually effaced towards their narrow free extremities.
These lateral lobes are rounded at their extremities, and flattened
in such a manner as to allow each lobe to slide easily under the lobe
immediately preceding. _Tail_ subtriangular, convex, equalling the
head in size, with the posterior termination rounded. On the centre of
its surface, when accidentally decorticated, a slight elevation may be
traced, if the specimen be held in a certain light, which appears to
be a continuation of the middle lobe; this extends to within a short
distance of the posterior angle of the tail, when it is either entirely
effaced or terminates in an abrupt truncation. Another elevation runs
parallel to and at a short distance from the edge of the tail. These
elevations are connected by obscure parallel lines, imitating the
spaces between the lateral lobes. When the tail is fractured on the
borders, a semilunar depression is visible, exhibiting concentric
striæ. The whole Surface of the animal has a jet black polish. Length
from 6 to 12 inches.
The original of our cast is in the cabinet of J. P. Wetherill, and was
found near Cincinnati, Ohio. It is of a yellowish colour, and occurs in
argillaceous slate. Specimens are common in most cabinets of American
fossils. The Lyceum in New York, possesses a fragment of an individual
of this species, which must have been at least 17 inches long. Our
cast, No. 22, is from the gigantic tail in the cabinet of P. A. Brown,
Esq. Mr. Stokes describes the I. gigas as a new species under the name
of Asaphus Platycephalus, in Geolog. Trans. vol. i. N. Series. His
specimen was found in the limestone of. St. Joseph's, Canada.
Isotelus Planus.[38] _Dekay._ Cast No. 23.
[Footnote 38: The general usage of naturalists is to prefix a short
Latin caption to the species which they discover--but as some
authors do not follow this fashion, we are satisfied to suffer their
descriptions to stand without it. We believe, indeed, that the time is
not very distant, when every author will be expected to publish his
discoveries in his vernacular tongue.]
_Head_ more rounded than the preceding, and less elevated. _Tail_
flat, rounded. Total length two inches and one-tenth. Breadth one
inch and one-tenth. Length of the head, six-tenths--of the abdomen,
eight-tenths, and of the tail seven-tenths.
Dr. Dekay is of opinion that this species may possibly prove to be the
young of the preceding. The relative proportions of its buckler and
tail vary considerably from those of the I. gigas; and the depth of
the lateral lobes, which exceeds three-tenths of an inch, would almost
of itself determine it to be a new species. The original, from which
our cast was taken, is in the cabinet of J. P. Wetherill. It was found
near Newport, Kentucky, and occurs in argillaceous slate. The fossil is
of a dirty yellow colour.
Isotelus Cyclops.[39] _Green._ Cast No. 24. Fig. 7.
[Footnote 39: From the Greek for "round eyes,"]
Clypeo antice attenuato, plano; oculis rotundis, proximis; cauda
ovata, acuminata.
The head of this species is much more elongated than it is in the
two preceding species. The anterior portion of the buckler is much
prolonged. The eyes are approximate, rounded, and near the posterior
edge of the head. The abdomen is furnished with eight distinct
articulations; the middle lobe is scarcely broader than the lateral
lobes; tail rather broader than the head, and ovate; posterior
termination more rounded than the buckler. Length nearly three inches.
The specimen from which our model was taken belongs to the Albany
Museum. No label is attached to it, but I was informed by Mr. Meach,
one of the proprietors, that it was found in the western part of the
State of New York. It is embedded in an ash-coloured limestone. The
specimen is a good deal worn--but the peculiar form of the eyes, and
the narrowness of the middle abdominal lobe, clearly distinguish it
from either of Dr. Dekay's species.
Isotelus Megalops.[40] _Green._ Cast No. 25.
[Footnote 40: From the Greek for "great eyes."]
Clypeo antice subrotundato, postice arcuato; oculis magnis,
rotundis, eminentissimis; cauda suborbiculari, limbo lato;
articulis abdominis octo.
The buckler in its contour resembles very much the head of the I.
gigas; it is, however, rather more rounded before, and arcuated behind.
The oculiferous tubercles, are very peculiar, being very large, round,
and exceedingly prominent. They have a good deal the appearance of
solid hemispheres placed on the forehead of the animal. They are
exactly on a line with the two abdominal furrows. The abdomen is
composed of eight distinct articulations; the middle lobe is rather
larger than the lateral lobes. The tail is suborbicular, convex, and
rather less than the head. Length nearly five inches. Breadth almost
three inches.
This magnificent Isotelus was obtained near Trenton Falls, in New York,
by P. A. Browne, Esq., and now forms a part of his fine collection
of fossils, in this city. It occurs in black transition limestone.
It differs essentially from the I. gigas of Dekay, in the magnitude,
collocation, and contour of the eyes. Those of the I. gigas are oblong
and lunate, and nearly half the distance between the anterior and the
posterior edges of the buckler; those of the I. Megalops are not only
much larger, but they are round, and very near the posterior border of
the head.
Isotelus Stegops.[41] Green. Casts Nos. 26 and 27.
[Footnote 41: From two Greek words, which signify "covered eyes."]
Clypeo antice, caudaque postice attenuatis; cute coreacea punctis
minimis.
The head of this species is nearly in the form of a spherical triangle;
its anterior edge is vertically flattened all round, but does not
produce a narrow raised rim, such as is described by Dr. Dekay, to
belong to the I. gigas. The eyes are prominent, and rather nearer the
lateral edge of the buckler, than to its posterior border. The shell
of the buckler forms a remarkable projection over the top of each
oculiferous tubercle, something like an eye-lid. Continuous with the
edge of this cuticular projection, there is a curved linear depression,
which terminates on both sides, at the edge of the buckler. This
kind of suture, though remarkably developed in this species, is not
peculiar to it, being more or less distinct in most of the Isoteli.
The articulations of the abdomen are lost; there can be little doubt,
however, that they were eight in number. The tail is subtriangular, and
less in magnitude than the buckler.
This fossil is among the number of fine specimens in the cabinet of
J. P. Wetherill. It is in a rolled or contracted attitude, and is
somewhat distorted. We have given, however, models of the head and
the tail, in two distinct pieces. The external shell, or calcareous
covering, is more perfect in this specimen than in any other we have
ever seen. A considerable portion of the under side of the anterior
part of the buckler, is also well preserved, and perfectly coincides
with the figure and description given of it by Dr. Dekay and Mr.
Stokes. There is another fragment of an Isotelus in the cabinet of Mr.
Wetherill, showing eight articulations of the abdomen, which probably
belongs to another individual of this species. The dorsal shell is in a
high state of preservation. This species is embedded in clay slate, and
was found in Newport, Kentucky.