MAZATLAN BIVALVES 119 



dorsali, prope cardinem perangulato, interne semper plus 

 minusve crenato; margine ventrali planato seu incurvo ; 

 superficie modo liratd, liris plus minusve divaricantibus, modo 

 striatd, seu omnino Icevi ; color e pu rp ureo, ad marginem 

 ventralem viri di: ligamento curto, lato, effosso. 



Variat t. omnino viridi, planata, liris tenuibus. 



"Crenarum indoles Modiolam sulcatam, Lam. aliasque 

 species in mentem vocat ; statura similis est M. Lavalleano, 

 jyOrb. (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 

 a few 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, long. '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. 



