122 MAZATLAN BIVALVES 



Modiola Brasiliensis, Hani. Desor. Cat. p. 234. DJcr. in lit. 



Mytilus bicolor, Brug. Cat. 



Modiola Guyanensis, Lam. An. s. Vert. vol. vii. p. 20, no. 4. 



Deles. Rec. i. 13, f. 9. 

 Mytilus Guyanensis, Desk. Enc. MetJi. vers. vol. ii. p. 565, 



no. 24. 

 = Modiola semifusca, Sow. (non Lam.) Gen. f. 6. Rve. Conch. 



Syst. pi. 101, f. 6. C. B. Ad. Pan. Shells, p. 251, no. 399. 



The typical form is remarkably constant in its characters, 

 while the ?var. is very changeable. Shell extremely thin, 

 very rhombic, with the ventral and dorsal lines very straight, 

 at an anle 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 light orange, separating the 

 two areas. Inside stained dorsally with reddish purple. 

 Umbos not decorticated. Long. 3 '3, lat. 1'5, alt. 1*1. 

 Hob. Brazils, Guyana, DunJcer. Venezuela, (Porto Cabello 



et Caraccas,) Bay of Guayaquil ; Dr. Tarns, teste Dunker. 



Mazatlan ; rare ; Lpool fy Havre Coll. Panama, rare, 



C. B. Adams. 



Tablet 558 contains 2 specimens, young and old. 



171 1. MODIOLA ?BSASILIENSIS, var. MUTABILIS. 



4. Modiola semifusca, P. P. C., Cat. Prov. ; non Soiv. ; nee 

 Lam. An. s. Vert. vol. vii, p. 22, no. 11, (teste Hani.) 

 Jtf. ? Brasiliensis, t. solidiore, epidermide atro-fusca induta, 

 apicibus plerumque decorticatis ; statura minor e : forma ma- 

 xime variante ; margine ventrali recto sen valde incurvato ; 

 plus minusve elongata ; angulo diagonali indistinctiore ; mar- 

 gine posterior e 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. semifusca. 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 



