86 Mr. W. King on certain Genera 



Atrypa undata, and in the Camerophorias, they curve in and eon- 

 join at their upper margin so as to form an arch more or less 

 resembling that of Pentamerus. 



With reference to the suspending plate of the Pentameruses, 

 its position, and its connexion with the arch or condyle plates, 

 establish its identity with the mesial plate, which serves to di- 

 stinguish certain genera and certain species. In Spirifer cris- 

 tatus, S. Walcotti, S. rostratus, Zeiten, Martinia imbricata, &c, 

 this plate, which is large, is situated between and independent 

 of the condyle plates ; in Strigocephalus it is a well-known in- 

 ternal appendage ; in Spirifer heteroclitus it is largely developed 

 in comparison with the condyle plates, which are cemented to 

 the lower part of its sides ; in Orthis adscendens and the Came- 

 rophorias it is comparatively small, and attached to the crest of 

 the arch as in Pentamerus ; in certain Orthises, and in the Lep- 

 tsenas, it exists under a rudimentary form, projecting a little 

 below the central line of their remarkably flattened arch-shaped 

 process*. 



tened arch, separated from the roof of the deltidial valve as in Pentamerus 

 conchidium, but not suspended as in this species by a mesial plate. The 

 arch is so flat and the natis of the opposite valve passes so close up to its 

 under surface, especially in old specimens, as to leave little or no opening 

 for a pedicle; indeed I suspect that this part only belonged to young indi- 

 viduals. I am not acquainted with the armature of the opposite valve of 

 Uncites, it is therefore difficult for me to form any positive conclusion as to 

 its generic affinities. In the synoptical table it is placed in the family Te- 

 rebratulidce, on account of its resemblance to Pentamerus conchidium in a 

 tew particulars. 



* The most remarkable internal structure that I know of is to be seen in 

 the dorsal valve of a shell labelled " Terebratula concentrica from the Eifel," 

 specimens of which I owe to the kindness of M. de Verneuil and Mr. W. 

 R. Loftus. In this species the condyle plates are attached to a process, 

 which, to use a homely comparison, resembles a shoe-lifter. Imagine a pro- 

 cess of this kind, about a third of the length of the shell, with its narrow 

 end fitting into the rostral point, and its lateral margins attached to the in- 

 side of the dorsal valve along its medio-longitudinal region ; then imagine 

 the superior margin of the condyle plates attached to its under or convex 

 surface, one on each of, and along, its sides, and a tolerably correct idea 

 will be formed of this singular apophysis. To complete the internal structure 

 of this shell, I may add that its lower valve is furnished with a deep mesial 

 plate, which supports a concave crural base, and that it possesses a pair of 

 spiral appendages, — the latter character added to its external form is in favour 

 of this shell belonging to Atrypa : whether it should be made to form another 

 genus I am not at present prepared to offer a positive opinion. Notwith- 

 standing its dissimilarity to all other known Palliobranchs 

 in its internal structure, I am led to suppose that the appa- 

 ratus of the dorsal valve of this species is a modification of 

 those condyle plates (in many Spirifers) which are drawn 

 in towards each other at their superior half as here repre- 

 sented : what is required to convert such a pair of condyle 

 plates into the apophysis of Atrypa concentrica is the approximating parts 



