ANCYLUS. 
217 
handle of a cover ; in which case it should be written 
Ansulus 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. hi. 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 Limnceidce, 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 
