14 COiXKLIN. [Vol. XIII. 



itself again, though so far as I could observe it never voluntarily 

 leaves the shell upon which it is carried about. It is also said ^ 

 to be found in numbers on blades of eel grass, though I have 

 not seen it in such positions. 



C. adunca, a species abundant along the Pacific coast, is 

 remarkably like C. convexa in size, shape, and color of shell, 

 as well as in habits and development. Keep, in his West 

 Coast Shells, says of it : "The most common species is C. 

 adunca Sby., hooked slipper shell. The apex is strongly 

 recurved, giving the shell a hooked appearance. Its color is 

 brown, but the deck is white. Living specimens may often 

 be found growing upon rocks or upon other shells. Common 

 length from one-half to three-fourths of an inch. Abundant." 

 Mr. Harold Heath, who has been kind enough to send me 

 specimens of this species, together with material for a study of 

 its embryology, writes me that individuals are found in about 

 equal numbers upon the shells of the " black turban " (Chloro- 

 stoma funebrale), and upon shells inhabited by hermit crabs. 

 "The individuals found upon the ' black turbans ' seem to come 

 to sexual maturity earlier than upon the hermit shells. Several 

 times on pulling off shells of adunca from the 'black turbans,' 

 I was surprised to find eggs under very small shells, very much 

 smaller than are found with eggs on the hermit crab shells." , 

 It seems to me that we have here a case parallel with C. plana 

 and its dwarf variety, though the difference between the two 

 forms in C. adunca is very much less striking than in the case 

 of C. plana. That the phenomena in the two species are simi- 

 lar is still further borne out by the fact that the average 

 number of eggs laid by each individual of C. adunca found 

 upon the "black turbans" is 173.3, while the average number 

 laid by those on hermit crab shells is 201.1. Concerning the 

 habits of C. adunca, Mr. Heath writes : " Their shape indicates 

 that they never leave the spot to which they first become' 

 attached. Sometimes surrounded by Bryozoa, the shell is clear 

 within the Crepidula shell. Still, when taken off, they can, 

 and sometimes do, regain a foothold. Many that I placed 

 loose in the aquarium have attached themselves to the ' black 



1 Cf. Gould : The Invertebrata of Massachusetts. 



