File:The Quarterly journal of the Geological Society of London (12734172774).jpg

T. DAVIDSON AND W. KING ON THE TRIMERELLID^I. 147
lines

hinge depressed, or concave in the middle

cardinal facet
large

passing down with a considerable inclination to the cardinal
buttress, from which it is separated by a well-defined edge

plat-
form extending to about half the length of the valve

strongly-
depressed along its surface

with perpendicular lateral walls

vaults
extending to a little way underneath the central part of the hinge

cardinal buttress low

standing approximately at a right angle to
the plane of the shell

as wide as the platform

umbonal chambers
extending to some distance under the hinge outside of the deltidium.
Brachial valve deeper and more convex than the opposite one

hinge
raised in the middle

with a strong lamelliform cardinal process

platform well developed

a little longer than that of the opposite
valve

its vaidts of considerable depth

median plate much elongated.
Mr. Billings, to whom palaeontolgists are indebted for the dis-
covery of this species, and who was the first to give a brief descrip-
tion of it, accompanied by a figure of a fragment of the internal cast
of the ventral valve, informs us that the largest specimen he has
seen is three inches and a half in length, by three inches in width.
He also states " there is sufficient to show that this species is quite
distinct from T. grandis. If a section were made across the beak
of a perfect shell of T. acuminata, it would show four perforations
arranged in a curve, exactly as in the similar section of the Swedish
species figured by Dr. Lindstrom. But if the beak of T. grandis
were cut across, it would show only two orifices, and they would
be the homologues of the two lateral perforations .our umbonal
chambers'. in the section of T. aciiminata, because in T. grandis the
two central tubes .our platform-vaults'. do not extend into the
beak, but terminate before they reach it."
According to our mode of describing its interior, the species
under description shows six "perforations" — four in the pedicle-,
and two in the brachial valve. It is extremely difficult to give all
the " perforations " in one view

our figure 4 d , PI. XV. only shows
those (vaults) belonging to the platforms

there are, however, in the
pedicle-valve of the specimen represented other two (umbonal
chambers), one on each side of the umbo.
The deltidium is usually wide and concave, giving the area a
singular appearance. The hinge is remarkable in having its
middle in the pedicle-valve concave, and the corresponding part in
the opposite valve convex

the convexity of the latter part is in-
creased by the lamelliform cardinal promioence (PL XV. fig. 4 d ). The
cardinal facet exhibits the lozenge faintly, though with close inspec-
tion its bordering lines may be made out. There are two small
" umbo-lateral scars," one at the entrance, and on the outer side,
of each umbonal chamber (fig. 1, x, PI. XII.). The scars on
the platform and crescent, though far from satisfactorily defined
(PI. XV. fig. 4"), appear to be formed and arranged as represented in
PI. XII. fig. 1.
T. acuminata occurs chiefly in the state of casts

• ,

in the Guelph

We have succeeded in making some very good gutta-percha moulds from
some of these