MARINE MOLLUSCA OF THE UNITED STATES. 15 



Family SPIRULID.E. 

 This family differs from those preceding it in having a calca- 

 reous spirally coiled and concamerated shell. There is but one 

 recent genus, Spirula^ and only one species. 



Genua SPIRULA, Lamarck. 

 Extr. de Cours. 1799. 



1. S. Peronii, Lamarck. Figs. 14, 15. 



Anim. sans Vert., vii. GOl. 1823. 

 Spirulafragilis, Stimpsou, Check List, 6. 18G0. 



Shell white, nacreous, coiled in two or three turns, which do not 

 touch each other, something like a ram's horn. The surface ex- 

 hibits constrictions, at short intervals, each of which corresponds 

 to an internal partition, so that the whole shell is divided off into 

 chambers, having a tube at one side, so that the whole are in com- 

 munication. Inhabits the o^jen sea all over the world, and is cast 

 upon the shores during storms. 



Class GASTEROPODA. 



Head distinct, furnished with e3'es and tentacles. Body usually 

 protected by a conical or spiral shell. Lower part of body formed 

 into a thickened, expanded, creeping disk or foot. 



Order L PROSOBRANCHIATA. 



Animal creeping or swimming, protected b}' a shell usually large 

 enough to contain it. Branchige plume-like, situated in advance of 

 the heart. Sexes distinct. 



A. Siphonostomata. Animal provided with a siphon and having 

 a canaliculated shell. Carnivorous. The shell is spiral, and its 

 axis is usually imperforate. Operculum lamellar, horny. 



B. Holoi^tomata. Respiratory siphon wanting, or represented 

 by a mere lobe in the collar of the mantle. Shell spiral or limpet- 

 shaped, generally somewhat globular, and with an entire rounded 

 aperture. The gills are plume-like, placed obliquelj^ across the 

 back, or attached to the right side of the neck. These mollusks 

 inhabit the sea or fresh water, and all of the latter, as well as a 

 portion of the former, are phytophagous. 



