MAZATLAN BIVALVES 139 
190. ByssoARcA MUTABILIS, Sow. 
Proc. Zool. Soc. 1833, p. 17. 
Area mutabilis, Rve. Conch. Ic. pl. 13 f. 85.—Hanl. Deser. 
Cat. p. 156, pl. 18, f.52.—B. M. Cat. D’Orb. Moll. p. 82, 
no. 733.—C. B. Ad. Pan. Shells, p. 259, no. 418. 
Compare Arca 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. ; 
Natal, B. M.)—A very similar, perhaps identical species is 
from Australia, Jukes. 
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 umbos. Teethin B. mutabilis very numerous, 
asin 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, lat. 1°35, 
alt. °72. 
Hab.—Isle of Plata, under stones, Cwiming.—Ecuador, D’ Orb- 
tgny.—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. 
