REPTILIAN AGE. JURASSIC PERIOD. 105 



Class GASTEROPODA. 



Subclass PULMONIFERA. 



Order Iiioperculata, 



Suborder LIMNOPHILA. 



Family LIM^tEIDJE. 



Shell dextral or sinistral, thin, and horn-colored, varying from elongate- 

 subcylindrical to ovate or discoidal ; capable of receiving the entire ani- 

 mal when retracted ; aperture simple ; columella with or without a fold ; 

 lip sharp. 



Animal with a short dilated muzzle; tentacles short and compressed, 

 or elongate and slender, bearing the sessile eyes at their mner bases. 

 Mantle with its margin simple or very rarely digitate. Mouth armed with a 

 corneous upper mandible ; lingual teeth numerous, arranged in transverse 

 rows, the central minute and the lateral uncinate. Respiratory orifice 

 on the right side, and the vent at the left of the neck. Foot ovate or 

 lanceolate. 



The above diagnosis is framed so as to include three subordinate groups, present- 

 ing in their various species a wide range of forms, and more or less important 

 differences in the characters of the animal. These sections or subfamilies may be 

 characterized as follows: — 



1. Liiniiiaeiuae. 



Shell spiral, dextral. Animal with tentacles short and flattened, or triangular. 

 Includes Limncea, and probably Chilina and Amphipeplea. 



3. Pliysinae. 



Shell sinistral, otherwise much as in the Limnceince, Animal with tentacles elongate and slender. 

 Includes Pliysa, Physopsis, Aplexa, Macrophysa,^ and Camptoceras. 



3. PlanorlbmaB. 



Shell involute, discoidal, dextral, or sinistral^? aperture more or less crescentic. Animal with tentacles 



as in the PhysincE. 

 Includes Planorbis Taphius, Bathyomphalus, Gyraulus, and Segmentina. 



Notwithstanding the striking differences of form observed in the shells here 

 ranged under the sections of this family, the animals of these several types agree 

 in so many respects, that conchologists generally place them together in one family; 

 while some also include Ancylus, Acroloxus, Latia, and Gundlaclda. 



' Macrophysa, Meek. The type for which this name is proposed is the curious Eocene species 

 described by Deshayes (Coq. Foss. II, 90; X, 11 and 12) under the name Physa columnaris. It 

 is a remarkably elongated, subcylindrical shell, with a deep suture, and a comparatively very small 

 body whorl. Its aperture is oval and small, or less than one-third the entire length of the shell, 

 angular behind, and subangular or abruptly rounded in front ; columella smooth, flattened, and some- 

 vrhat twisted. It seems to be intermediate between Camptoceras and Aplexa, and may possibly be 

 ranged as a subgenus under the latter. 



" See note, p. 106. 



14 December, 1864- 



