MARINE MOLLUSCA OF THE UNITED STATES. 119 
Family LIMAPONTIIDA. 
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 Elysiide, 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 conferve in small pools above half-tide. 
1. L. zonata, Girard. 
(Wiobe.) 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 bands. 
Boston Harbor, Mass. 
OrvER IV. PTEROPODA. 
* Head indistinct, with two wings on the sides of the mouth , gills 
internal, body inclosed in a shell. 'Thecosomata. 
Family Cavouinipa. 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 CympuLtup#. 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, rarely wanting. 
Family Limacinip™. 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 distinet; 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 Cxiionip&%. Animal fusiform; head with a series of 
conical prominences on each side ; wings two. 
