THE SHELL-FISH OF THE COAST. 31 
very common (cylindrical) small shell that is found 
almost everywhere among the tide-water sedges or 
grasses. The latter is the Melampus bidentatus (PI. 
1, fig. 2), an air-breathing or pulmoniferous mollusk, 
like the common garden-snail, but requiring a cer- 
tain amount of salt water for its happiness. Why 
this should be necessary is not exactly known, the 
animal breathing by means of a true lung, as in 
the case of all true land-snails (Pulmonata). The 
slipper-limpets are readily recognized by their some- 
what boat-shaped shells, which are found either 
loose by themselves, or attached to other shells and 
stones, their outlines being largely modified accord- 
ing to the receiving substance. This modification 
extends in such varying directions that it is not yet 
clear what proportion of the ordinarily accepted spe 
cific characters is founded upon it. For the pres- 
ent, however, we recognize some three or four spe- 
cies as occurring on our coast: Crepidula fornicata 
(Pl. 1, Fig. 15), the largest form, which not rarely 
measures an inch and a half, or more, in length; 
Crepidula glauca or convexa (Fig. 25), a small humpy 
shell, whose presence appears to be generally asso- 
ciated with that of the small hermit and of Nassa 
obsoleta ; and Crepidula plana or unguiformis (Fig. 26), 
with a nearly flat shell, which is frequently found 
within the apertures of other shells. The slipper- 
limpets have the habit of crowding upon themselves, 
or of ‘bunching,’ so to speak; they may accord- 
ingly be picked up in accumulated masses, and this 
is especially true of the more northerly shores. 
Closely related to the preceding is the crucible- 
