106 MAZATLAN BIVALVES 
can be separated from the British, or the aberrant forms from 
each other. The general aspect of the shells from the two 
localities is sufficiently distinct; the Pacific shell being gen- 
erally thinner and rather flatter ; but the specimens from the 
Canaries brought by R. M’Andrew, Esq. so remarkably coin- 
cide with those from Mazatlan that Prof. Forbes was unable 
to distinguish them. 
The same changes of form observable in the Atlantic shells 
obtain in the Pacific. Sometimes it is very broad, with the 
ventral margin produced and flattened ; sometimes orbicular, 
with all the edges well rounded. Sometimes (like its fresh- 
water neighbour Cyclas cornea) it is considerably flattened, 
sometimes very much swollen. The umbos vary in the same 
manner; sometimes being but slightly prominent, sometimes 
very tumid and projecting, occasionally capped like Cyclas 
calyculata. The texture also is by no means constant; being 
sometimes hyaline and perfectly transparent, sometimes of a 
dull ashy colour, sometimes opaque white. One valve, agree- 
ing in other respects, displays a large honeycomb pattern, the 
* eells being transparent on an opaque ground. Most unexpect- 
edly of all, differences are found in the hinge. The small cardinal 
teeth are sometimes distinct and conical, sometimes they pass 
off gradually into the laterals. These are sometimes long and 
slender, sometimes short, strong, slightly truncate. Some- 
times the ligament pit is concealed ; sometimes very conspicu- 
ous while the adjacent teeth are small. Often one or both of 
the cardinal teeth are absent (probably through accident,) and 
the laterals are scarcely perceptible ; at other times they are 
unduly prominent. The aspect of the shell is generally slightly 
iridescent, either smooth or with extremely faint strie of 
srowth, with a very thin rather glossy epidermis. Almost all 
the Mazatlan specimens were found nestling (not boring) in 
family groups, in holes and crevices of the large Chame, 
Spondyli, &c.; the finest and most characteristic individuals 
lurking among mixed animal and vegetable debris, in the bur- 
rows of dead Lithophagi, Petricole, &e. Most of the specimens 
found were very young, but displayed their characters accur- 
ately under the inch or half-inch achromatics. They were 
frequently found in the small tubes of worms where they could 
never arrive at maturity. Jf specimens had been taken from 
the large hollows of dead bivalves, their favourite haunt in 
this country, they would probably have been stronger and 
larger. As the species is known to have existed from the 
