Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/403

1869.] MM. Coquand, Hébert, Pictet, de Verneuil, E. Renevier, and others, as to the real or supposed age of the rock which, at the Porte-de-France, contains the Terebratula viator (diphya), Pictet. I may also observe that, with the exception of MM. Hébert and Lory, the geologists above named seem to consider the deposit in question referable 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. Hébert 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. Hébert has placed it.

When at Geneva (on the 16th of February), M. Pictet showed me his interesting series of Diphyoid Terebratulœ, 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 distinguish 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 subparallel 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 concave 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 elaborate memoir on the "Terébratules du groupe de la T. diphya, 1867," M. Pictet furnishes us with a numerous series of figures in which all the principal modifications in form are faithfully represented.

In 1830 Link proposed to distinguish the Diphyoid shells by the generic denomination of Pygope, which view was afterwards advocated by Prof. King ; but all the palæontologists who have subsequently 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