MAZATLAN BIVALVES 87 



GENUS CHAMA, (Pliny J Linn. 

 121. CHAMA FEONDOSA, Brod. var. MEXICANA. 



Chama frondosa, var. 1. Brod. Proc. Zool. Soc. 1834, p. 149. 



Zool Soc. Trans, vol. i. p. 302, pi. 38, f. 1, 2. Rve. Condi. 



Ic. pi. 1, f. 1 b.B. M. Cat. D'Orb. Moll. p. 87. no. 773. 

 Compare Chama echinata, Brod. in Zool. Beech. Voy. p. 150, 



pi. 43. f. 9. (? = C. echinata, pars, Brod. Proc. Zool. Soc. 



1834, p. 150: Trans. Zool. Soc. 1835, p. 305, pi. 39, f. 57. 



Rve. Conch. Ic. pi. 7, f. 35, jun. C. B. Ad. Pan. Shells. 



p. 254, no. 407.) 



The Mazatlan Chamse were sent in tolerable numbers, and 

 yet so commonly were their surfaces abraded that it is difficult 

 to discriminate the specific characters. The bulk of them are 

 certainly C. frondosa, var. b, of Brod. and Rve., but were con- 

 sidered a distinct species by Messrs. Cuming and Hanley. 

 The typical C. frondosa comes as far north as S. W. Mexico 

 (P. P. C.) but was not found among the Mazatlan shells. I 

 have not ventured however to describe it as a distinct species, 

 till more is known of its power of local variation. 



Shell when extremely young (as shewn by examining the 

 umbos of the smallest valves found, under a half-inch achrom.) 

 somewhat Venus-shaped, with concentric ridges, and a very 

 finely shagreened surface, occasionally passing into very fine 

 radiating stria? : interior as in adult. Many of the shells do 

 not display this structure, but pass at once to the next stage, 

 generally with vaulted spines, sometimes with irregular laminae. 

 The inner margin at an early state is not crenulated : colour 

 white, gradually developing a rich pink. In its adolescent 

 condition, the interior is of a rich plum purple, irregularly 

 diffused, more or less stained with orange, especially at the 

 teeth. The margin during this period often displays a most 

 minute set of crenulations, within the line of meeting of the 

 valves. All the specimens found in this state had lost their 

 external characters. When properly adult, it is generally cover- 

 ed with very irregular concentric layers rising into lamellae, 

 rarely into spines, with a tendency to broader foliation in a pos- 

 terior radiating area, ill defined. The interior crenations become 

 then almost extinct. Colour outside brownish red, within 

 white, more or less stained with the same. Shell attached 

 sideways, generally by about a third of its surface, with very 

 large adductors, and hinge teeth which are more or less ser- 

 rated ; white, bordered with purplish brown. The ligament 



