I N V BRTEBRATE PAL^3( >NTOLOGY. 307 



varices behind as the shell advanced in growth : inner lip very thin, or want- 

 ing : columella smooth ! 



The type of this group has the general aspect of a smooth, slender 

 Tritonifusus, but differs from that and the allied groups in the peculiarity 

 of forming, at irregular intervals, internal varices, by 1 lie thickening and den- 

 tieulation of the interior of the outer lip: the varices being left behind as the 

 shell increased in size, and not absorbed. Between these internal thicken- 

 ings, the lip was always thin and sharp. Another peculiarity is the almost 

 entire absence of an inner lip. 



In the presence of internal varices, this type agrees with that for which 

 I have proposed the name Trachytriton ; but it differs materially in surface- 

 markings and texture, and to a less extent in form. Some very similar spe- 

 cies have been referred to Pterodonta and Tylostoma by foreign palaeontolo- 

 gists ; but our shell, and the foreign species alluded to, differ widely in form 

 and general appearance from the typical oval species of Pterodonta, with a 

 dilated outer lip, such as P. elongata and P. ovata, d'Orbigny ; while, accord- 

 ing to Dr. Stoliczka, Tylostoma has no canal, or even well-marked anterior 

 sinus. 



Dr. Stoliczka has described from the Cretaceous rocks of India (Pa- 

 Ueont. Indica, II, 42), under the name Pterodonta I terebralis, a shell that 

 agrees with this type so nearly in form, surface-characters, and even in the 

 possession of denticulate internal varices, that I should not for a moment 

 hesitate to adopt the conclusion that the two species are congeneric, were it 

 not for the fact that he says his shell has two plaits on the columella. As 

 he states, however, that the plaits are obsolete at the aperture, and I have not 

 yet been able to see the columella of our type quite far enough around within 

 the aperture, it is still possible that it may possess this character also. If so, 

 its agreement with the Indian species would apparently be exact in every 

 respect except in mere specific characters. However that may be, I cannot 

 for a moment believe that such slender fusiform shells, which we have no 

 reason whatever to believe had the lip in the least expanded, can properly be 

 referred to the genus Pterodonta. Indeed, Dr. Stoliczka himself expresses 

 the opinion, on page 294 of the same work, that his species may even prove 

 to belong to the Pyramidellidm or to the Cerithiidce. 



So far as known, this type seems to be confined to the Cretaceous. 



