162 DEVONIAN FAUNA. 



in so complicated a genus, the species of shells which are in such a poor state of 

 preservation, and doubtfully refers them to his Carboniferous Eli. pleurodon. 1 In 

 this he has been followed by Davidson and others ; but it seems probable that 

 there are sufficient grounds for distinguishing them from the Carboniferous shell, 

 even in their defective state. Rh . pleurodon generally has five ribs 011 the fold, 

 though they may vary from three to nine. From the study of a large number of 

 specimens of our Pilton shell I find that they have almost invariably four ribs on 

 the fold with three on the sinus. Gosselet lays great stress on the number of ribs 

 on the fold, using it as a distinguishing character for his various Upper Devonian 

 species ; and, if his view of its importance is correct, we have here a sufficiently 

 definite specific distinction ; for, even granting that in many species of Rhyncho- 

 nella the number of ribs on the fold is very various, there seems no reason why 

 in other cases their definite number should not be a point of specific importance. 

 Moreover, the profile of the present shell seems much less rounded than that of 

 Rh. pleurodon ; the front of the median ribs of the dorsal valve and of the lateral 

 ribs of the ventral valve sweeping definitely outwards till they are abruptly turned 

 in at the margin at a sharp angle, more in the manner of Rh. anisodonta than of 

 Rh. pleurodon. 



Our shells occur as casts, and as I have only seen specimens of the true 

 Rh. pleurodon retaining the shell, I have been unable to compare the interior, the 

 arrangements of which are well indicated in our fossils. These arrangements 

 appear exactly the same as those by which Hall and Clarke 2 define their genus 

 Camarotoechia, to which they refer Glosselet's Upper Devonian species, though they 

 do not indicate whether they also include in it Rh. pleurodon. 



While, however, there seem to be sufficient grounds for distinguishing Rh. 

 Parlridgids from Rh. pleurodon, their relationship is probably very close, the former 

 being apparently somewhat more specialised than the latter. One point of agree- 

 ment which may be noted is the slight median channeling of the lateral ribs, which 

 is often seen in the Pilton shells, and is mentioned by Davidson as a character of 

 Rh. pleurodon. 



Affinities. None of the Upper Devonian Rhynchonellae described by G-osselet 3 

 in 1887 appear to correspond with our species. While several have the same 

 number of median ribs, they all (as well as Rh. Daleidensis, F. Homer, 4 or Rh. 

 inaurita, Sandberger 5 ) differ in their more rounded profile and the less acutely 

 produced sweep of their medio-dorsal and latero-ventral ribs. 



1 1836, Phillips, ' Geol. Yorks.,' vol. ii, p. 222, pi. xii, figs. 2530. 



2 1894, Hall and Clarke, ' Pal. N. T.,' vol. viii, pt. 2, p. 189. 



3 1887, Gosselet, ' Ann. Soc. Q-eol. Nord.,' vol. xiv, p. 188. 



4 1844, ~F. Eomer, ' Ehein. Uebergangsgeb.,' p. 65, pi. i, figs. 7 a c. 



5 1856, Saudberger, ' Verst. Ehein. Nassau,' p. 337, pi. xxxiii, figs. 5 5 c. 



