MAZATLAN BIVALVES 139 



190. BYSSOABCA MUTABILIS, Sow. 



Proc. Zool. Soc. 1833, p. 17. 



Area mutabilis, Sve. Condi. Ic. pi. 13 f. 85. Hani. Descr. 



Cat. p. 156, pi. 18, f. 52.^. M. Cat. D'Orb. Moll. p. 82, 



no. 733. C. B. Ad. Pan. Shells, p. 259, no. 418. 

 Compare Area Americana, D'Orb. (non Gray,) B. M. Cat. 



D'Orb. Moll. p. 80, no. 714 (Brazils) : B. M. Cat. Cuba Moll. 



p. 43, no. 521. ?=A. imbricata, Brug. (Jamaica, C. B. Ad. ; 



]N"atal, B. M.) A very similar, perhaps identical species is 



from Australia, Juices. 



Shell greatly resembling the European B. tetragona, but dif- 

 fering from it as follows. Epidermis in B. tetragona hairy, 

 even on the angular ridge ; in B. mutabilis sublamellose, as in 

 B. Pacifica, with the lamellar portions gill-like. Ligament in 

 B. tetragona scarcely shewing over the area, but with numer- 

 ous diamonds between the umbos ; in B. mutabilis darkly 

 diffused over the whole area, with or without one or two dia- 

 monds at the unibos. Teeth in B. mutabilis very numerous, 

 as in B. Pacifica ; in B. tetragona larger and fewer, somewhat 

 remote. Posterior portion in B. mutabilis strongly ribbed ; in 

 B. tetragona, like the rest of the shell. It is distinguished from 

 the young of B. Pacifica, which it very much resembles in 

 form, by the posterior ribs and the epidermal ridge along the 

 posterior angle. Pedal gape generally large. In young shells 

 the structure under the glass is very beautiful. I am unable 

 to find any constant character by which the West Indian 

 specimens can be separated from it, though the shape some- 

 what differs ; and the smaller ribs are more imbricated. The 

 same form is from Natal, B. M. Among the best characters to 

 distinguish Byssoarks are the form and nature of the ligament, 

 the hinge teeth, and the epidermis. The markings and outline, 

 as well as the inter-umbonal space, often vary considerably 

 in the same species. The more constant characters are how- 

 ever often overlooked in descriptions. Long. *8, lot. 1*35, 

 alt. '72. 



Sab. Isle of Plata, under stones, Owning. Ecuador, Tf Orb- 

 igny. Panama and Taboga ; not uncommon under stones 

 and in the crevices of rocks, near low water mark ; C. B. 

 Adams. Mazatlan ; rare : L'pool Col. 



Tablet 649 contains 7 specimens differing in age and shape. 



