ANCYLUS. 



217 



handle of a cover ; in which case it should be written 

 Arfsulus or Ansylus. 



The shell differs from Siphonaria (with which alone 

 it can be confounded, on account of the peculiar form 

 of the muscular scar, and the lateral situation of the 

 apex,) in being thin and pellucid, only finely striated, 

 and covered with a thin olive periostracum. 



It only agrees with Patella in the outward appear- 

 ance of the shell; for in that genus the apex is anterior, 

 and in this it is posterior, as in most Univalves. 



This animal has been moved from one family, and 

 even order, to another, as naturalists have settled 

 among themselves whether it breathed by gills or 

 lungs. Rang places it with the Pleurobranchi, and 

 observes that it lives on stones and aquatic plants, 

 but that he never observed it to breathe free air. 

 Mr. Guilding (ZooL Journ. iii. 335.) and Treviranus 

 (Journal Phys. 1832, t. 17.), who published a detailed 

 dissection of the genus, mistake the valve which 

 closes the opening of the breathing cavity, for a gill. 

 The head is quite destitute of the labial appendages 

 noticed by Rang. 



Mr. Berkeley (and my own observations bear out 

 his accuracy) observes that the animal is undoubtedly 

 one of the Limnceida, and nearly allied to Physa. 

 The pulmonary cavity, like that of Physa, is on the 

 left side, with a valvular margin, in one corner of 

 which is situated the rectum ; between this and the 

 foot is the orifice of the matrix. They are herma- 

 phrodite, and may be observed in connection, as was 

 observed by Lister (Anim. Ang.\ about the end of 

 September ; and, as the latter author affirms, they fix 



