MAZATLAN UNIVALVES 269 
My attempts to find specific differences between the Atlantic 
and Pacific shells have entirely failed. The former are generally 
of a more reddish, the latter of a browner cast; but those 
from Chili belong to the Atlantic type; while those from 
Honduras go through the same changes as the Mazatlan shells. 
The Patagonian sheils may belong to either type. The C. 
echinus of Brod. represents the form in which all the ribs are 
equal and very spiny; the C. hystrix that in which a few are 
developed, with large spines, at the expense of the rest. The 
two forms run into one another, and into the common form 
almost imperceptibly. In first describing them, Brod. candidly 
states that he would not be positive that they are not all varie- 
ties of C. aculeata. There is a distinct variety which bears 
the same relation to the typical form that C. squama does to 
C. nivea. It is flat, very regular, without spines, but covered 
with extremely crowded imbricated scales. The Californian 
variety is the most aberrant, being small, nearly round, and 
never spiny. It might pass for a distinct species, were it not 
that a few of Mr. Nuttall’s specimens exactly belong to the 
Mazatlan type, while some few of the degenerated Mazatlan 
specimens are closely allied to those from Monterey. — The 
young shells which Menke obtained from coral on Spondylus, 
Chama, and Murex nigritus, and affiliated to the New Zealand 
form, (so well marked that it received the same name from three 
distinct sources), appears from the diagnosis to belong to this 
species. 
C. aculeata belongs to the group of regularly spiral Cre- 
pidule. Ut begins life as a smooth, glossy, light horn-coloured, 
Velutina-shaped shell, with rapidly imereasing whirls, and 
a sunken apex. This is much larger than the nuclear part of 
C. nivea, though the adult shell is much smaller ; being about 
‘025 across when it begins its second stage. C. nivea begins 
with concentric ridges and goes on smooth, rapidly increasing, 
but in a regular curve. C. aculeata makes a sudden expansion 
when forming its deck, and then develops ridges as in the 
form C. Lessonii. These soon become more or less undulated, 
and then assume the state of vaulted spines, differing in size 
and arrangement, and in the period at which they commence. 
At the same time various rays or spots of chesnut colour 
appear. Within, the growth of this species offers a well 
marked contrast to that of C. nivea and its congeners. Instead 
of forming a basal columella lip and then throwing up a deck 
at a considerable angle, this shell makes the columella lip the 
