MAZATLAN BIVALVES 65 



that Cythereae may always be known by the umbonal markings, 

 that in this species they are extremely variable : the white 

 rays take very different directions : or it is not rayed at all : 

 or the white is entirely absent. The colour varies from dark 

 lustrous brown to light grey : sometimes uniform, often biradi- 

 ate with dark brown, generally more or less spotted in the 

 young shell ; occasionally tinged concentrically with red, very 

 rarely with purple or greenish, but never with the reddish 

 chesnut which is very characteristic of D. chione. The inside 

 also is often stained with violet, occasionally with yellow. The 

 principal difference from D. chione is the shape, which is more 

 swollen, almost subangulated in the posterior dorsal portion, 

 the angular line generally ending in a produced posterior 

 margin. These characters however are not constant. There 

 is a spotted variety rather hard to distinguish from its W. In- 

 dian analogue, D. maculata, not being less tumid (as Sow. 

 states,) but rather more, and losing the posterior angulation. 

 The pallial sinus in D. chione is generally denned by a some- 

 what broader line, and is a little more pointed ; though I found 

 the shape vary not a little in the many hundred specimens of 

 D. chionaea which I have carefully examined. 



The East Indian specimens (if Philippi's figures are correct) 

 have the ligament rather shorter, the lunule longer, and the 

 pallial sinus less angulated. In other respects they exactly 

 agree, and are considered identical by the very accurate Desh. ; 

 not, of course, for geographical reasons, by C. B. Ad. If dis- 

 tinct, they will take Koch's name of D. elegans. 



If I have rightly affiliated certain very small valves, the 

 young shell (of which the smallest measures only *02 across) 

 first developes the marginal teeth, leaving the central ones till 

 it gets older. The anterior portion of the shell also is of abnor- 

 mal size. 



The longest specimen measures long. 2*77, lot. 3*58, alt. 1'6. 

 A transverse specimen 1*94, 2*54, 1*14. 



A rounded specimen 1*62, 1*87, 1*03. 



Hob. St. Elena, in sandy mud, 6 fm., Cuming. Ecuador ; 

 St. Elena, D'Orbigny. Taboga, very rare, C.B.Adams. 

 La Paz, Lieut. Green -S. W. Mexico, P. P. (7. Found 

 abundantly at San Bias and Mazatlan, Lieut. Belcher. 

 Mazatlan, common ; L'pool <$[ Havre Col. (D. elegans) 

 Philippines, Cuming. Swan Eiver, N. H,, Philippi. 

 Tablet 272 contains 11 small perfect and imperfect valves 

 which probably belong to this species. 



