64 APLYSIIDA. 
The Aplysiide, as a whole, are among the most modified Tecti- 
branchs. None of the existing genera approach the primitive con- 
dition of the family. No fossil forms are known. 
ANALYTICAL KEY TO GENERA OF APLYSIID. 
a. Anterior ends of pleuropodial (dorsal) lobes well separated, the 
lobes mobile and freely separable, at least in front; shell thin, 
with but little lime; genital orifice in front of the gill. Ex- 
ternal integument not warty. Lateral teeth with long cusps, 
serrate on both sides, Aplysvine. 
b. Rhinophores (posterior tentacles) situated near the middle 
of the space from anterior ends of dorsal lobes to the front. 
tentacles ; genital orifice under edge of mantle, |. Ternys. 
bb. Rhinophores small, situated between anterior ends of dor- 
sal lobes; mantle posterior, the genital orifice in front of 
and not covered by it, II. PARAPLYSIA. 
aa. Anterior ends of pleuropodial lobes contiguous, separated only 
by the genital groove, the lobes not freely mobile or readily 
spread outward; external integument usually warty or rough- 
ened. 
b. Genital opening well in front of the main mass of the gill ; © 
radula with a wide central tooth, and narrower laterals 
with several denticles, Dolabrijerine. 
c. Mantle covering gill, at least in part; dorsal slit. 
mainly or wholly behind middle of the animal’s 
length ; sole of foot broad. 
d. A small but well-developed shell present; back 
of animal convex. 
e. Body widest behind middle ; no opening in 
the mantle exposing the shell; gill largely 
uncovered by, and projecting beyond, the 
mantle, III. DonaABRIFERA. 
ee. Body widest near the middle; a large open- 
ing in mantle exposing part of the shell, 
IV. PETALIFERA. 
dd. Shell wanting ; body very flat; teeth tricuspid, 
blunt, V. PHYLLAPLYSIA. 
ce. Gill not in the least covered by mantle; dorsal slit 
subcentral; sole narrower than the body; shell a 
minute vestige or absent ; body plump, 
VI. Norarcuvus. 
