_Shell dark greenish-gray, with narrow, longitudinal, serrated ribs:
radii absent: scutum with the adductor and articular ridges forming a
cavity, which runs up to the apex of the valve._
_Hab._--Cape of Good Hope; Algoa Bay; attached to sandstone and to
Patellæ; Mus. Brit., Cuming, and Stutchbury.[108]
[108] I have seen three separate lots of this species all from the
Cape of Good Hope; one lot was collected by Dr. Krauss, at Algoa
Bay, and I strongly suspect is the species described by him in his
'Südafrikanischen Mollusken' as _Conia porosa_. If the species,
figured by Chemnitz, and mentioned in a note (p. 329), under _T.
porosa_, be the present species, the specimens probably did not
come from Tranquebar, on which point Chemnitz speaks only from
memory. I have seen one specimen ticketed New South Wales, it is
possible, considering the case of _T. rosea_, that this may be
correct, but I should like to have further confirmation before
giving it as a habitat.
_General Appearance._--Colour dark greenish gray; form steeply
conical; surface covered, especially in the lower half of the shell,
by numerous, narrow, sharp, longitudinal ridges, but slightly
prominent, and serrated or transversely divided into small teeth:
when the shell has been much disintegrated, the upper part of the
surface consists of the exposed, smooth, rather large, upfilled
parietal tubes. I have seen no instance of the development of
the radii; sometimes even the sutures are with great difficulty
distinguishable, though I believe they always reach the outer
surface; sometimes the sutures are wide from the disintegration of
the edges of the compartments. Orifice rounded or oval.
_Scuta._--The scuta and carinal half of the terga are blueish-green.
In the scuta neither the articular ridge or furrow are much
developed: the adductor ridge is prominent, and is united to the
articular ridge, about half way up the latter, thus forming a rather
large, triangular cavity, which runs up to the apex of the valve.
The _Terga_ are beaked. The spur, measured across the upper part,
is half as wide as the valve; it is bluntly pointed; it is placed
quite close to the basi-scutal angle of the valve, so that there
is no basal margin on that side; it curves towards the scutum, its
extremity extending beyond the basi-scutal angle.
_Structure of the Shell and Radii._--The parietal tubes are rather
large, especially those adjoining the inner lamina of the walls. The
shell is of singularly little specific gravity, which is due to the
parietal tubes not being filled up with shelly matter nearly to so
great an extent as in the other species; even in the uppermost part
the tubes are not solidly filled up, only their external sides are
thickly coated with greenish-black shell, which by corrosion becomes
grayish. The radii, as stated, are not developed: the shell breaks
with singular facility along the sutures, and the radii are then seen
to be most feebly represented by a few very small branching ridges.
The alæ have their edges plainly crenated. The sheath is dark green,
with the lower edge free.
The _Mouth_ presents no particular characters. With regard to the
_Cirri_, I am doubtful whether any confidence can be placed in the
numbers of the segments being constant; but I may state that the
second cirrus contained thirteen and sixteen segments in its two
rami; the third cirrus only fourteen in both rami; and the sixth
cirrus twenty-six segments in both rami. Whereas in every specimen
of _T. porosa_, the longer ramus of the third cirrus contained more
segments than either ramus of the second. About half the segments,
namely, those in the middle of both rami of the third cirrus, are
furnished with coarsely and doubly pectinated spines, like those in
_T. porosa_.
_Affinities._--Upon the whole, this species is more nearly allied
to _T. porosa_ than to any other. In the cavity formed by the
union of the adductor and articular ridges, it is allied to _T.
cœrulescens_. This species differs from all, in its little specific
gravity, consequent on the parietal tubes being only slightly filled
up, and in the peculiarly serrated, narrow, approximate ridges on
the external surface of the walls. The character derived from the
adductor ridge, just alluded to, is remarkable. In the shape of the
terga, in the absence of radii, and in the structure of the body,
this species approaches closely to _T. porosa_.