Notes on Lingual Dentition of Mollasca. 291 



A figure of the shell, with descriptions of the external charac- 

 ters of the animal, will be found in Land and Fresh-water Shells 

 of North America, Part II., p. 73-74. As there has been some 

 discussion in regard to this species having two pairs of eyes, we 

 will here repeat that the eyes are situated in the place usual in 

 the Limna?id«. 



Linsrual membrane broad, with 22.1.22 teeth. Central teeth 

 upright, narrow, widening and knobby at the base ; apex re- 

 curved, and produced into an obtuse beak. Laterals nine on 

 each side of the central line, in a straight transverse row, wide, 

 quadrate, apex recurved, prolonged beyond the base of the tooth 

 in a more or less broad blunt beak. Uncini about thirteen on 

 each side of the median line, in oblique transverse rows, not 

 attached to a plate, simple and not recurved ; the first eight from 

 the extreme lateral edge of the membrane long, narrow, arm- 

 shaped, terminating in a wrist-Hke contraction and hand-like 

 expansion, strongly digitate. The remaining uncini gradually 

 changing into the shape of the laterals, but still not merging into 

 them, the line of demarcation being strongly marked. 



There is great variation in the beak-like projection of the re- 

 curved apex of the laterals, and still more in the digitation of 

 the uncini. 



The jaw is long, narrow, slightly arcuate, with blunt ends; 

 anterior surface smooth. 



By the characters of its lingual dentition, and its horny jaw, 

 Pompholyx appears nearly related to Planorhis. 



Fig. 9, a, represents the central and two laterals to the right of 

 the median line ; h gives one full series of uncini to the left of the 

 median line ; and c one of the uncini in profile. 



Plaiiorbis trivolvis, Say. 



The specimen from which was extracted the lingual mem- 

 brane here figured was collected at Newport, Khode Island, by 

 Mr. Sam. Powel. 



Lingual membrane broad, with slightly curving rows of teeth. 



