Marine Shells of Massachusetts. 277 
becoming more elongated, as it increases in size... The 
color is a uniform white, both within and without, the 
outer surface of the old shell losing the purity of its 
color from exposure. — . 
The largest individual, which I have, i is 14 in. in 
length, and one inch in breadth. — 
This species appears to be always parasitic. "The 
shells of the Pyrula canaliculdta, and Pyrula cárica, 
Lam. are often covered externally with the C. Sornicata, 
Say, and upon the fauces of the aperture with both the 
C. fornicáta and the C. plana, while the interior is oc- 
cupied by a hermit crab. "The Natica duplicata, Say, 
and the valves of the Pecten concéntricus, Say, are 
often found loaded with these species in the same man- 
ner.. Some of the finest specimens, which I have found, 
were upon the inner surface of the crust of the ert 
lus polyphémus, Linn., (Horse Shoe, or King Crab 
company with the C. fornicàta, while the latter hela 
exclusive possession of the outer surface. 
This species cannot, (the question has been raised) be 
the C. fornicata, blanched by seclusion from the light, 
for although it is never found on the outer surface of 
shells, it is accompanied on the inner surface by the 
C. fornicata, which here retains its colors.* The latter is 
also very obviously distinguished, except from the young 
of the former, by its convexity, and by its margin, which 
is in a uniform curve with the disc of the. shell, instead 
of being abruptly d deflected as in ! lana. 
Say observes that this species i : 
of the United States from Florid 
* A large s on 
face several al of the C. fornicàta. Le : 
