68 PERMIAN FOSSILS, 



the hinge of the rostral or perforate valve (PI. XX, figs. 6 a, 10 A; PI. X, fig. 8 e), 

 and acting on a pair of sockets excavated in the hinge-plate of the opposite valve 

 (PI. XX, figs. 7 a,\lB; ?l. X, fig. 8/). 



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 Cleiothyris pectinifera .- they approximate more or less 

 to each other at their superior margin in Trigonotreta, and some other genera (PI. X, 

 fig. 28 a) ; in Pentamerus, Orthisina, Camerophoria (PI. VIII, fig. 3 a), they conjoin 

 superiorly, forming an arch-shaped process more or less acute ; while in Leptana 

 (PI. XX, fig. 6 b) 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 Trigonotreta 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 point of the beak (PI. VIII, 

 figs. 3 and 4 b). The ventral median plate, for such this last part may be termed, is seen 

 independent of the dental plates in Belthyris cristata (PI. VIII, fig. 1 8 a), and without 

 them in Strigocephalus (PL XIX, fig. 1 a) : it is protruded within the arch in Spirifer 

 heteroclytus,wA within the saucer-shaped process inLeptcena analago (PL XX, fig. 6 c). 

 The ventral median plate evidently subserves different purposes in different genera : 

 it supports the arch in Pentamerus and Camerophoria, and answers as a muscular 

 fulcrum in Belthyris, 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 certain species of 

 Productus. The singular 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 tfes«k 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. 11 C). The socket-ridges are seen in Belthyris cristata 



^ Vide Geology of Russia, vol. ii, pi. v, fig. 2 b, d, 



2 Annals and Magazine of Natural History, vol. xviii, p. 86. 



