MARINE MOLLUSCA OF THE UNITED STATES. 107 



Family TRIOPID^E. Mantle small, edged with tentacular ap- 

 pendages ; gills on the middle of the hinder part of the back, in 

 a common cavit3 r surrounding the vent ; vent dorsal. 



* OilU various, not arranged round the vent but usually in rows 

 along the sides of the body. 



Family TRITONIID^E. 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 DOTONID^E. Tongue narrow ; teeth in a single central 

 series ; tentacles sheathed at the base, retractile ; gills superficial, 

 fusiform, on the sides of the back. 



Family JEOLIDID^E. 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 HERM^ID^E. 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- 

 tinguish this family from jEolididx, and the presence of papillose 

 gills from Elysiidse. 



Family ELYSIIDSE. 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 LIMAPONTIID^J. Body depressed; tongue narrow; 

 teeth in a single, central series ; tentacles none, or simple, con- 

 tractile ; gills none external. 



Family DORIDID^E. 



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: 



