122 MAZATLAN BIVALVES 
Modiola Brasiliensis, Hanl. Deser. Cat. p. 234.—Dékr. in lit. 
Mytilus bicolor, Brug. Cat. 
Modiola Guyanensis, Lam. An. s. Vert. vol. vil. p. 20, no. 4.— 
Deles. Rec. t. 13, f. 9. 
Mytilus Guyanensis, Desh. Enc. Méth. vers. vol. ii. p. 565, 
no. 24. 
= Modiola semifusea, Sow. (non Lam.) Gen. f. 6.—Rve. Conch. 
Syst. pl. 101, f.6.—C. B. Ad. Pan. Shells, p. 251, no. 399. 
The typical form is remarkably constant in its characters, 
while the Pvar. is very changeable. Shell extremely thin, 
very rhombic, with the ventral and dorsal lines very straight, 
at an angle of about 30°, a well marked diagonal ridge, and a 
somewhat straight posterior line, at a mean divergence of about 
105° from the dorsal line: with concentric furrows on the 
posterior part of the shell. This is coloured olive green ; while 
the anterior ventral part is smooth, and of an orange colour, 
with a narrow diagonal! band of lght orange, separating the 
two areas. Inside stained dorsally with reddish purple. 
Umbos not decorticated. Long. 3°3, lat. 1°5, alé¢. 1°1. 
ffab.—Brazils, Guyana, Dunker.—Venezuela, (Porto Cabello 
et Caraccas,) Bay of Guayaquil; Dr. Tams, teste Dunker.— 
Mazatlan; rare ; JL’pool § Havre Coll.— Panama, rare, 
C. B. Adams. 
Tablet 558 contains 2 specimens, young and old. 
1716. Moptota ?BRASILIENSIS, var. MUTABILIS. 
Modiola semifusea, P. P. C., Cat. Prov.; non Sow.; nec 
Lam. An. s. Vert. vol. vii, p. 22, no. 11, (teste Hanl.) 
M. ? Brasiliensis, ¢. solidiore, epidermide atro-fuscd induta, 
apicibus plerumque decorticatis ; staturad minore: forma ma- 
xime variante; margine ventrali recto seu valde incurvato ; 
plus MAINUSVE elongata ; angulo diagonal indistinctiore ; mar- 
gine posteriore plus minusve excurvato. 
This shell suits exactly the description of Lam., but is 
distinct from the shell which Mr. Hanley states to be the 
true M. semifusea. According to Hanley, Dunker, and the 
first impressions I received from an examination of the speci- 
mens, this may be a rough water var. of M. Brasiliensis: there is 
however sufficient doubt to make it desirable that the varia- 
tions from other recorded localities should be accurately 
