MAZATLAN BIVALVES 65 
that Cytherez 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 defined 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. chionza 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, lat. 3°58, alt.1°6. 
A transverse specimen ,, Bde op De OM arrestin Alone topuwe bees 
A rounded specimen ss fy Mids GBs bi ST ae 
Hab.—St. Elena, in sandy mud, 6 fm., Ouming.—Ecuadar : 
St. Elena, D’ Orbigny.—Taboga, very rare, C. B. Adams.— 
La Paz, Lieut. Green—S. W. Mexico, P. P. C.—Found 
abundantly at San Blas and Mazatlan, Lieut. Belcher.— 
Mazatlan, common; L’pool & Havre Col.—(D. elegans) 
Philippines, Cuming.—Swan River, N. H., Philippi. 
Tablet 272 contains 11 small perfect and imperfect valves 
which probably belong to this species. 
