1869.] DAVIDSON ^DIPHYOID TEKEBRATtJLiE. 307 



MM. Coquand, Hebert, Pictet, de Verneuil, E. Kenevier, and 

 others, as to the real or supposed age of the rock which, at the Porte - 

 de-France, contains the Terehratula viator (diphya), Pictet. I may 

 also observe that, with the exception of MM. Hebert and Lory, the 

 geologists above named seem to consider the deposit in question re- 

 ferable either to the Jurassic period or, rather, to the stage termed 

 Tithonian, which some geologists consider to be intermediate in 

 age between the Neocomian and Jurassic, while M. Hebert still 

 correlates it with the lowest stage of the Neocomian, his " calcaire a 

 Ammonites macilentus" The difference in opinion seems, however, 

 to be gradually disappearing, and I believe that the limestone of the 

 Porte-de-France will finally be left where M. Hebert has placed 

 it*. 



When at Geneva (on the 16th of February), M. Pictet showed me 

 his interesting series of Diphyoid Terebratulce, assembled from various 

 localities, and at the same time pointed out the differences, with 

 which I was already acquainted, and which appeared to him to dis- 

 tinguish the Cretaceous from the Jurassic (?) form. 



Thus, in the shell from the Porte-de-France {Ter. viator, Pictet) 

 there exists in the larger or ventral valve a regularly subpa- 

 rallel fold, commencing at the extremity of the truncated beak 

 and extending to the central portion of the anterior frontal margin, 

 and that whether the deviating lateral halves of the valves remain 

 permanently apart, or become again conjoined in front so as to leave 

 a circular hole in the middle of the shell. In the cretaceous T. 

 diphyoides, on the contrary, the same fold has a longitudinal or con- 

 cave depression along its middle. In the smaller or dorsal valve 

 there exists in T. viator a depression or concave sinus, commencing 

 at the umbonal beak and extending to the frontal margin, while in 

 T. diphyoides the same sinus has along its middle a narrow rounded 

 elevation. There are also several other minor differences, though I 

 must confess that, apart from these peculiarities, there exists a strong 

 resemblance between T. viator and T. diphyoides ; consequently it 

 is necessary, in order to be able to discriminate between the two 

 species, to attend to the differences above specified. In his ela- 

 borate memoir on the " Terebratules du groupe de la 2\ diphya, 

 1867," M. Pictet furnishes us with a numerous series of figures 

 in which all the principal modifications in form are faithfully re- 

 presented. 



In 1830 Link proposed to distinguish the Diphyoid shells by the 

 generic denomination of Pygope, which view was afterwards advo- 

 cated by Prof. King f ; but all the palaeontologists who have subse- 

 quently written on the subject have preferred leaving these shells in 

 the genus Terebratula. I had likewise, in 1848, informed Prof. 

 King that the loop in T. diphyoides (and no doubt in T. diphya 

 and viator) was short,, and much resembling in shape and character 

 what we find in T. vitrea, T. carnea, etc. And since the interior 



* See "Notes on Continental Geology" in the April, May, June, and July 

 numbers of the Geological Magazine for 1869. 



t A monograph of the Permian Fossils of Great Britain. 



