462 T. Davidson — Tertiary Brachiopoda. 



del Progno o Torrente d'lllasi, etc., p. 19. Yerona, 1850. Ter. con- 

 torta (Schaft). Schauroth Yerszeichniss der Yerst. Herzogl Natur. 

 Zu Coburg, p. 165, 1865. 



This is a very important and exceedingly variable species, and its 

 correct identification offers certain difficulties, and especially so when 

 we compare it with a similar-shaped shell from the Miocene to 

 which Sig. Michelotti had in 1839 applied the specific designation of 

 Buchii. Some palaeontologists would seem inclined to unite the two 

 into a single species, while others would appear disposed to retain 

 them as distinct. Ithynchonella polymorpha is so variable, that two or 

 three so-termed species might be made out of its different shapes 

 did we not possess all the intermediate and connecting links. It is 

 very irregular or inequilateral, and is either a little wider or shorter 

 than long, broadest anteriorly, tapering at the beaks, and has always 

 one half of each of its valves twisted indifferently either to the 

 right or to the left, so that one half of each valve is near the front 

 either higher or lower than the other; no regular median fold or 

 sinus exists, the frontal line being during half its length convex and 

 elevated, or bending suddenly downwards by a rapid curve. The 

 beak is angular, with a small circular foramen under its extremity, 

 which is surrounded and more or less widely separated from the 

 hinge-line by a deltidium. The surface of the valves is in some 

 specimens entirely smooth, while in other examples five or more 

 short angular or rounded ribs occur in the vicinity of the margin on 

 the one half of the shell only, the other half remaining smooth, 

 but in other individuals both sides of the shell possess these ribs, 

 while the posterior half of the shell is usually smooth. The largest 

 example I have seen measured ten lines in length by six in depth. 



Through the kindness of several friends I have been able to 

 assemble a large number of the Eocene Rh. polymorpha and of the 

 Miocene Rh. Buchii, so that I was able to select from them the very 

 instructive series of specimens figured in our plate. I believe it 

 will be seen from these drawings that if not specifically identical, it 

 is highly probable tliat Rh. Buchii is a regular descendant, with 

 small modifications due to time and altered circumstances, of the 

 more ancient Rh. polymorpha, and that the Miocene shell much more 

 rarely presents the short ribs which are so often met with in speci- 

 mens of Massalongo's species. It may, therefore, be advantageous 

 to preserve the two designations. Sig. Meneghini is of opinion that 

 the shell mentioned by Alcide D'Orbigny under the designation of 

 Rli. Zignodiana, and erroneously placed by him into his Terrain 

 Callovien (Prodrome, vol. 1, p. 343, 1850), is a synonym of the 

 species under description, as is likewise the case with the Ter. con- 

 torta of Schaftault, which was figured and illustrated by Baron 

 Scharauth in 1865, but erroneously referred by him to the Cretaceous 

 period. M. Mortillet describes Rh. polymorpha as a Terebratula 

 approaching Rh. incurva.^ Dr. Oppel has, likewise, described and 

 illustrated a Jurassic species under the designations of Rh. alta and 



^ Terrains du versant Italien des Alpes, compares a ceus du versant Francais, 

 Bull. Soc. Geol. de France, second series, vol. 19, p. 892. 



