belonging to the Class Palliobranchiata. 87 



The plates of the ventral valve, as they are prolongations of 

 the socket-walls, must be considered as identical with the socket- 

 plates to be seen in certain palaeozoic species, as Orthis eximia, 

 Spirifer cristatus, S. striolatus, Meckl., Hypothyris? (Terebratula) 

 nucella, &c, and which are characteristic of that singular Silurian 

 group described by Pander under the name of Porambonites. 



It requires to be mentioned, that in a great many of the shells 

 lately cited, I have cleaved the plates of the dorsal valve in the 

 same manner as it is usual to divide those of Pentamerus, which 

 proves that they are composed of two united lamellae. M. Ver- 

 neuil seems to be of opinion, that it is in Pentamerus alone that 

 the plates (at least the mesial one) possess a bilamellar structure, 

 and that this shell is therefore essentially distinguished from all 

 other palliobranchiate genera. In some of the shells that I have 

 broken up, the lamellae separate as freely as those of Pentamerus ; 

 in most they are not quite so easily divided, and in a few there 

 is some difficulty in separating them ; the difference, it is highly 

 probable, being simply due to the more or less intimate union of 

 the two lamellae of which they are composed. 



Strigocephaltjs. 



This genus possesses an area furnished with a deltidium, which 

 is open in young individuals and cicatrized in those fully grown ; 

 in individuals of an intermediate age, the cicatrix exhibits a small 

 circular opening, which resembles the entire subapical foramen 

 of Hypothyris obsoleta, &c. 



The inside of the dorsal valve is furnished with a mesial 

 plate, resembling that which suspends the arch in Pentamerus : 

 it extends from the umbonal cavity to within a third of its 

 length of the anterior margin of the valve, increasing in depth 

 as it advances. With the exception of two slight ridges running 

 into the condyles, there is no other vestige of an arch-shaped 

 process. 



In the ventral valve, a massive slightly curved process (the 

 concave side being upwards) stretches from the middle of the 

 hinge to a little behind the centre of the opposite valve, where 

 it clasps as it were the mesial plate by means of a bifurcated 

 extremity ; in other terms, this extremity is notched, which 

 actually enables the process to pass to a little more than an 



to become confluent as in this diagram, which represents a 

 transverse section of the apparatus enlarged. Another mo- 

 dification of the condyle plates is to be seen in Spirifer 

 mosquensis and S. rostratus (that is, the Jurassic shell so 

 named by Zeiten), which have them so much prolonged as 

 nearly to touch the frontal margin of the valve to which 

 they are attached. (Vide Geology of Russia, vol. ii. for the former species, 

 and Von Buch on Delthyris for the latter.) 



