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 a comb-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 Taenioglossa.. 



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. 



