MARINE MOLLUSCA OF THE UNITED STATES. 107 
Family Triopip#. Mantle small, edged with» tentacular ap- 
pendages; gills on the middle of the hinder part of the back, in 
a common cavity surrounding the vent; vent dorsal. 
* Gills various, not arranged round the vent but usually in rows 
; along the sides of the body. 
Family Trironup%. Tongue broad, teeth many in each cross 
series; jaws horny; gills superficial, fusiform, or branched on 
each side of the back; vent lateral ; foot linear, channeled. 
Family Dorontip#. Tongue narrow; teeth in a single central 
series; tentacles sheathed at the base, retractile; gills superficial, 
fusiform, on the sides of the back. 
Family Houmips. Tongue narrow; teeth in a single central 
series; jaws horny; tentacles subulate, simple, rarely ringed, con- 
tractile; gills superficial, fusiform, or branched on the sides of 
the back; vent lateral. 
Family Hermaip&. Body elongated, not provided with a dis- 
tinct mantle; mouth unarmed, or with corneous jaws, tentacles 
sometimes wanting; when present two, dorsal, non-retractile ; 
gills papillose; vent usually central, on the posterior half of the 
back ; genital orifice at the right side. 
The dorsal position of the vent and the indistinct mantle dis- 
tincuish this family from Molididex, and the presence of papillose 
gills from Hlystide. 
Family Etysup#. Body limaciform, clothed with cilia; tongue 
narrow; teeth in a single central series; tentacles subulate or 
linear, folded; eyes sessile, near the bases of the tentacles ; gills 
in the form of plaits or vessels radiating on the surface of the 
back ; vent central, dorsal on the hinder part of the back. 
Family -Limapontiupm. Body depressed; tongue narrow; 
teeth in a single, central series; tentacles none, or simple, con- 
tractile ; gills none external. 
Family DORIDID A&A. 
These animals are most attractive nudibranchs, and may be 
studied by placing them in glass reservoirs of salt-water, as they 
are by no means shy, but extend their tentacles and display their 
branchial plumes to great advantage. In this family the gills 
are retractile into a common cavity, and the mantle is very large, 
either entirely or almost covering and concealing the foot. 
