PTEROPODA. 239 
A common form, occurring at the surface in harbors 
north of Cape Cod, as well as many miles off shore, is Spiri- 
alis Gouldii Stimpson, the shell of which 
resembles a conical Helix. The largest 
form on the -eastern coast of North 
America, extending from New York to the 
polar seas, is the beautiful Clione papillon- 
acea of Pallas, which has a head and lin- 
gual ribbon. It is rare on the coast of 
New England, but abundant from Labra- 
dor northward. We have observed it 
rising and falling in the water between 
the floe-ice on the coast of Labrador. It 
igs an inch long, the body fleshy, with no 
shell, the wings being rather small. 
The larve of the Pteropods pass through =~ ; 
a trochosphere stage, being, as in Cavolina, Fa i 
spherical, with a ciliated crown. It after- 
wards assumes a veliger form. Fig. 179 represents a worm- 
like, segmented, Pteropod larva, the adult of which is 
unknown. In other genera the larve are annulated, resem- 
bling the larvae of Annelides. 
The Pteropods are, in some degree, a generalized type. 
They have a wide geographical distribution and 
a high antiquity; forms like Cavolina, viz.: 
Theca, Conularia, Tentaculites, Cornulites, 
etc., dating back to the paleozoic formation ; 
Theca-like forms (Pugiunculus and Hyolithes) 
occurring in the primordial rocks, 
Order 3. Gastropoda.—This great assemblage 
of mollusks is represented by the sea-slugs, 
limpets, whelks (Figs. 180-183), snails, and 
Fig. 179.—Ptero- Slugs. The head is quite distinct, bearing one, 
ee and sometimes, as in the land-snails, two pairs 
of tentacles, with eyes either at the bases, or at the ends of 
the tentacles, or, as in Trivia californica (Fig. 184), they 
are situated on projections near the base of the tentacles. 
‘All the Gastropods move or glide over the surface by the 
‘broad creeping-disk, a modification of the foot of the clam, 
