MARINE MOLLUSCA OF THE UNITED STATES. 15 
Family SPIRULID&. 
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. 
Genus SPIRULA, Lamarck. 
Extr. de Cours. 1799. 
1. 8. Peronir, Lamarck. Figs. 14, 15. 
Anim. sans Yert., vii. 601. 1822. 
Sptrula fragilis, Stimpson, Check List, 6. 1860. 
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 open sea all over the world, and is cast 
upon the shores during storms. 
Crass GASTEROPODA. 
Head distinct, furnished with eyes and tentacles. Body usually 
protected by a conical or spiral shell. Lower part of body formed 
into a thickened, expanded, creeping disk or foot. 
OrpEeR I. PROSOBRANCHIATA. 
Animal creeping or swimming, protected by a shell usually large 
enough to contain it. Branchiz 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. Holostomata. 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 obliquely 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. 
