68 PERMIAN FOSSILS. 
the hinge of the rostral or perforate valve (Pl. XX, figs. 6a, 10 4; Pl. X, fig. 8 e), 
and acting on a pair of sockets excavated in the hinge-plate of the opposite valve 
(Ol. ROC ess 7a, eb PE OX cies!) 
In many genera the teeth are attached to a pair of plates (Pl. X, fig. 8 a), which, 
in the phraseology of Von Buch, are called ‘‘les lamelles de soutien des dents ;” 
an expression which may be conveniently rendered into dental plates. The dental 
plates are nearly perpendicular in Clerothyris pectinifera: they approximate more or less 
to each other at their superior margin in Zrigonotrefa, and some other genera (PI. X, 
fig. 28a); in Pentamerus, Orthisina, Camerophoria (P\. VIII, fig. 3a), they conjoin 
superiorly, forming an arch-shaped process more or less acute; while in Leptena 
(Pl. XX, fig. 6 4) this process is so far modified as to become saucer-shaped. The 
dental plates, whether conjoined or separated, vary greatly in length: in Camerophoria, 
Cleiothyris, and some others, they do not pass far into the cavity of the shell; in 
Pentamerus they extend to a considerable distance; while in Zrzgonotreta Mosquensis 
they are prolonged nearly to the frontal margin of the valve to which they are 
attached.1 ‘ 
In certain genera, as Pentamerus and Camerophoria, the arch-shaped process is 
attached by its crest to the medio-longitudinal line of the ventral or rostral valve, 
by means of a vertical plate directed forward from the pomt of the beak (Pl. VIII, 
figs. 3and40). The ventral median plate, for such this last part may be termed, is seen 
independent of the dental plates in De/thyris cristata (Pl. VIII, fig. 18 a), and without 
them in Strigocephalus (Pl. XIX, fig. 1a): it is protruded within the arch in Sprifer 
heteroclytus, and within the saucer-shaped process in Leptena analago (Pl. XX, fig. 6 ¢). 
The ventral median plate evidently subserves different purposes in different genera : 
it supports the arch in Pentamerus and Camerophoria, and answers as a muscular 
fulerum in Delthyris, as proved by the scars which I have seen on it in a beautiful 
specimen of D. octoplicata in the possession of Mr. T. Davidson. The central vertical 
plate or muscular support in the attached valve of Criopus, is perhaps its homologue ; 
and the same may be suggested of the broad, elevated ridge covered with muscular 
scars to be seen in the medio-longitudinal region of the large valve of certam species of 
Productus. 'The smgular process, in the form of a shoe-lifter, described in my paper on 
the “‘ Palliobranchiata,’” as occurring in the large valve of Cleiothyris concentrica, and 
which is seen in another allied species from Bohemia, in Mr. Davidson’s collection, 
I am disposed to think is a modified form of the ventral median plate. 
Passing to the opposite or dorsal valve, the cavities in the hinge-plate serving as 
sockets for the teeth, or, in other words, the dental sockets, are each bounded inwardly 
by a ridge, which occasionally becomes somewhat prolonged, passing into the cavity of 
the shell (Pl. XX, fig. 11C). The socket-ridges are seen in Delthyris cristata 
! Vide Geology of Russia, vol. ii, pl. v, fig. 26, d. 
* Annals and Magazine of Natural History, vol. xviii, p. 86. 
