BRACHIOPODA. 201 



The internal structure of these species has never been carefully elucidated, 

 and it is a matter of great difficulty to obtain material suitably preserved for 

 the elaboration of the critical features of the hinge. Clean internal casts are 

 seldom found, and no instance of the silicification of the valves has come to 

 our notice. By careful serial sectioning, however, it has been possible to ascer- 

 tain with reasonable accuracy the character of the hinge. The teeth are 

 usually supported by short vertical lamellae ; the hinge-plate is quite small and 

 is composed of two broad, short lateral processes, which are divided, for a por- 

 tion of their length only, by a median incision extending to the bottom of the 

 valve but not forming an inceptive spondylium as in Camarot(ECHia. The 

 dental plates are large. There is but the barest indication of a median septum 

 in the brachial valve. The muscular impressions are small and not deep; 

 those of the pedicle-valve making an oval scar continued from the narrow ped- 

 icle-cavity ; those of the brachial valve being narrow, elongate and extremely 

 obscure.* The interior of the pedicle-valve frequently preserves the ovarian 

 pittings and vascular sinuses while the characters are but faintly retained on 

 the brachial valve. The development of these features seems to be of specific 

 or varietal value only, as they are more rarely shown in the European examples 

 of R. cuboides and are absent in R. Emmonsi, which is a more finely plicated 

 shell, possessing other internal structure here described. 



The characters described are distinctive, but that they also occur in such 

 allied species of the middle Devonian as R. procuboides, Kayser, R. primipilaris, 

 von Buch, and R. parallelepipeda, Bronn, we can only surmise from a similarity 

 of exterior. They are reproduced with a very slight development of the median 

 septum, in R. Grosvenori, Hall, of the St. Louis limestone. 



To shells of this nature may appropriately be applied the designation Hy- 

 POTHYRis, King, 1846. There may seem to be some objection to the adoption 

 of this term, which was introduced at an earlier date by Phillips f for certain 



* Davidson has ^ven, on plate ii, of his Supplement to the Devonian Brachiopoda, figures (19, 19a) of 

 an intei-nal cast which is referred to R. niboides, but it would seem to I.e an erroneous reference. There is 

 nothing in the figures which suggests this species, but it appears to repi-esent a concavo-convex shell with 

 an extended beak and strong flabellate muscular scare on both valves ; in many respects suggestive of a 

 species of EAroxrA. 



t Palteozoic Fossils of Cornwall, Devon, and West Somerset, p. 35. 1841. 



