MAZATLAN BIVALVES 119 
dorsali, prope cardinem perangulato, interne semper plus 
minusve crenato; margine ventrali plandto seu incurvo ; 
superficie modo liratd, liris plus minusve divaricantibus, modo 
striata, seu omnino levi; colore purpureo, ad marginem 
ventralem viridi: ligamento curto, lato, effosso. 
Variat t. omnino viridi, planata, liris tenuibus. 
“Crenarum indoles Modiolam sulcatam, Zam. aliasque 
species in mentem vocat; statura similis est M. Lavalleano, 
D Orb. (et M. Senegalensi=variabili, Krauss) sed satis ab eo 
differt,” Dkr. M. glomeratus, Gould, (San Francisco) is about 
the size of this species, but more resembles M. edulis in form, 
and has no crenations. 
This extremely changeable little shell might furnish materi- 
als for many species, if only a few picked specimens were 
examined ; but between the perfectly smooth and the deeply 
sulcated, the swollen and the flattened forms, there exists 
so regular a gradation that it is impossible to separate them. 
The young shell is shaped like Modiola. The smooth speci- 
mens closely resemble the young of M. palliopunctatus, but 
maybe- distinguished by the presence of at least two or three 
denticles at the angle of the hinge line, and generally by 
afew more at the umbos. These may often be seen in the 
closed shell by transmitted light. Frequently the entire 
hinge line, as well as the posterior margin, is crenated. The 
majority of young specimens are smooth, and of adults sul- 
cated; but often very minute specimens are plicated while 
those of (for the species) large size are smooth. The colour is 
generally purple, with a larger or smaller greenish portion 
near the byssus ; but sometimes the purple is curtailed to a 
small patch, or is absent altogether. The shell is then gener- 
ally flat, with fine, branching furrows: and might fairly be 
taken for a different and well-marked species, but for inter- 
mediate forms. The smallest specimen found measured scarcely 
‘02 across ; an unusually large one, Jong. 45, lat. 24, alt. °32. 
Some forms of the green variety might easily be taken for 
another species. Nevertheless they go through the same 
changes from nearly smooth to coarsely plicate, and often 
display purple at the commencement, or have purple at the 
margin. Occasionally a smooth purple shell suddenly changes 
to a plicate green one. The green shells are generally flatter, 
and often have the ribs somewhat nodulous; in which state 
they are known from the young of Septifer Cumingianus by 
the absence of hairs on the epidermis. 
