MAZATLAN BIVALVES 87 
Genus CHAMA, (Pliny,) Linn. 
121. CHama FRONDOSA, Brod. var. MEXICANA. 
Chama frondosa, var. b. Brod. Proc. Zool. Soc. 1834, p. 149.— 
Zool. Soc. Trans. vol. i. p. 302, pl. 38, f. 1,2.—Rve. Conch. 
Ie. pl. 1, f.16.—B. M. Cat. D’ Orb. Moll. p. 87. no. 773. 
Compare Chama echinata, Brod. in Zool. Beech. Voy. p. 150, 
pl. 48. f. 9—(P=C. echinata, pars, Brod. Proc. Zool. Soc. 
1834, p. 150 :— Trans. Zool. Soc. 1835, p. 305, pl. 39, f. 57.— 
Rve. Conch. Ic. pi. 7, f. 35, jun.—C. B. Ad. Pan. Shells. 
p. 254, no. 407.) 
The Mazatian Chame 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 strie: 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 lamine. 
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 lamelle, 
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 
