een 
THE NATURAL HISTORY SURVEY. 319 
tinguished by the simple mantle margin of the animal. It is 
found in the three regions of the area, but is not common ex- 
cept in a very few localities. 
Superorder Streptoneura. 
In this group the nerves called the ‘visceral loop,” being ad- 
herent to the body wall, shared in the torsion of the visceral 
hump, forming a figure-of-eight, the left cord crossing beneath 
the right. (Lankester.) 
ORDER CTENOBRANCHIATA. 
Animal breathing by acomb-like gill-plume (or ctenidium) 
which is situated on the right side of the body, the left cten- 
idium having become atrophied by the torsion of the visceral 
hump. The organs on the right side only are well developed. 
Sexes distinct. 
SUBORDER STREPTODONTA. 
Lingual membrane with curved teeth, generally broadly 
reflected. 
Superfamily Tenioglossa.. 
Radula provided with numerous rows of teeth, with three 
laterals on either side of a central tooth (formula 3-1-3). 
FAMILY PLEUROCERID/. 
Shell: Turreted, smooth or with rings and knobs; epider- 
mis olivaceous; aperture channeled. Operculum paucispiral. 
Animal; With a broad, short muzzle, which is not retract- 
ile; foot short, wide, angulated near the anterior end; mantle 
margin not fringed, plain; tentacles of medium size, cylin- 
drical; eyes situated on short peduncles united to the outer. 
sides of the tentacles; lingual membrane with 3-1-3 teeth, of 
which the central is wider than high, multicuspid, and the lat- 
erals are long and narrow and multicuspid. The first lateral 
is generally wider than the two outer teeth. The family is 
oviparous. 
Genus PLEUROCERA Rafinesque, 1819. 
Shell: With a long and conical spire; aperture rather 
small, the outer lip produced into a canal about the middle; 
columella without callosity. 
