ae 
Marine Shells of see: ^ 279 
In the young shell the outline is more AM the 
apex usually distinct from the margin of the aperture, 
and the internal colors penetrating to the outer surface. 
marine bodies. On the south shore of Nantucket 
dis common on the crust of the Limulus polyphémus, . 
(Horse-shoe). Several | individuals are often found 
growing upon each other in an imbricate manner. The 
margin of the aperture is adapted to the surface on 
which it grows. Hence the margins of individuals, 
which grow on the valves of a Pecten are scalloped. A 
valve of the P. concéntricus is in my cabinet, on which 
are two specimens, the apex of one of which is near the 
basal margin o Arcos valve, and the position of the other 
is exactly the reve re, i in a. of which the 
Pecten being muc F more prominent.on the basal half. 
Another specimen has quite prominent ribs, like the 
Pecten, abruptly commencing at one fourth of the dis- 
tance from the apex, and extending over the remaining 
three fourths to the margin, which is scalloped. 
This species is very numerous at Nantucket, but is 
not often found4iving above low-water mark. It occurs 
at New Bedford, where a vast quantity of the dead 
shells: cover a part of the margin of a small island to the 
m ofa foot. This species occurs "so at Chelsea 
c. convéxa, Fo Journ. ET ONA II. p. 227. 
This species, which is easily distinguished from the 
C. fornicáta, is very accurately described by Say. The 
S ud 
* 
- This species is always found growing on shells or | 
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