MARINE MOLLUSCA OF THE UNITED STATES. 119 



Family LIMAPONTIID.E. 



In this group of slug-like forms the branchial appendages are 

 altogether absent, or represented only by simple lobes or ridges on 

 the sides of the body ; the tentacles are linear, and not longitu- 

 dinally folded as in Elysiidoe, and the body is depressed. 



Genus LIMAPONTIA, Forbes. 

 Loudon's Mag. Nat. Hist., v. 979. 1832. 



Head elevated at the sides into two crest-like ridges ; eyes 

 large, sessile on the back of the head, in the centre of pale circu- 

 lar spaces ; mantle distinct. 



Gregarious, feeding on confervce in small pools above half-tide. 



1. L. zonata, Girard. 



(Niobe.) Proc. Bost. Soc. Nat. Hist., iv. 211. 1852. 

 . Less than a line in length ; separation of body and head not 

 distinctly defined ; pale-reddish with transverse white hands. 



Boston Harbor, Mass. 



Order IV. PTEROPODA. 



* Head indistinct, with two icings on the sides of the mouth ; gills 

 internal; body inclosed in a shell. Thecosomata. 



Family Cavolinid^e. Animal with two united fins without 

 any posterior foot-like appendage between them; abdomen volu- 

 minous; gills in pairs ; shell calcareous, symmetrical, elongate or 

 globular. 



Family Cymbuliid.e. Animal globular or ovate ; fins two, ho- 

 rizontal, opposite, on each side of the mouth, with a small inter- 

 mediate lobe ; shell cartilaginous, slipper-shaped, rarety wanting. 



Family LiMACiNiDiE. Animal elongate, spiral; mouth at the 

 union of the two fins and intermediate lobe, with two small labial 

 swellings; fins elongate, rounded and united at their base by an 

 intermediate lobe bearing an operculum; mantle large, open in 

 front, forming a large gill cavity; gills internal; shell spiral, 

 transparent ; operculum distinct, spiral, vitreous, of few whorls. 



** Head distinct; wings two, or four, at the junction between the head and 

 the body, with a central intermediate lobe or rudimentary foot ; gills ex- 

 terior; no shell. Gymnosomata. 



Family CLiONiDiE. Animal fusiform ; head with a series of 

 conical prominences on each side ; wings two. 



