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 E. 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 

 cells 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 striae of 

 growth, with a very thin rather glossy epidermis. Almost all 

 the Mazatlan specimens were found nestling (not boring) hi 

 family groups, in holes and crevices of the large Chamse, 

 Spondyli, &c. ; the finest and most characteristic individuals 

 lurking among mixed animal and vegetable debris, in the bur- 

 rows of dead Lithophagi, Petricolse, &c. 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. If 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 



