MAZATLAN BIVALVES 115 



and by the rich olive epidermis, covering the umbos, and rising 

 into irregularly corrugated folds, which are very close' on the 

 posterior part. The interior displays a very dark purple over 

 the greater part of the surface. Anterior adductor very near 

 the margin. The outline varies considerably, but not so much 

 as in the next species. 



The largest specimen measures long. 2 '2, lat. 2 '38, alt. 1'26. 



A transverse 17, 2'05, 1'06. 



An inflated 1'62, 1'8, 1'16. 

 Hob. Mazatlan ; not common ; L'pool Col. 



Tablet 505 contains 3 specimens much produced ventrally, 

 slightly so posteriorly. 506, 3 do. less produced. 507, 3 do. 

 very slightly produced, normal shape. 508, 2 do. margin 

 regular, very' transverse. 509, 2 do. less transverse. 510, 3 

 do. very irregular outline. 511, 1 do. orbicular, aberrant, 

 approaching aberrant forms of the next species. 



165. CYBENA MEXICANA, Brod. and Sow. 



C.t." C. olivacece" simulante, sed ventricosiori, forma maxime 

 variante, suborbiculari, subtrigond, seu elliptic a ; parte posticd 

 seu prolong aid, sew subcarinatd, seu rotundatd ; extus albescente, 

 umbonibus plerumque maxime erosis ; epidermidefusco-olivaced, 

 lamellis creberrimis, non cor rug at is , plerumque detritis ; 

 intus albd, marginibus plus minusve violaced ; dent. card, 

 magis incequalibus, lat. magis extantibus ; sinu pallii minima, 

 marginibus paulum divergentihus. 

 Cyrena varians, P. P. C. Cat. prov. 

 Pars = Cyrena Mexicana, Zool. Journ. vol. iv. p. 364. Hani. 



Descr. Cat. p. 94. B. M. Cat. Corbiculada, p. 260, no. 59. 

 Var. = Cyrena altilis, Gould, Mex. fy Cal. Shells, p. 27, pi. 16, f. 5. 

 Jun. = Cyrena fragilis, Desk. ms. in Mus. Cuming. 

 ? = C. Floridana, Conr. Proc. Ac. N. S. Phil. iii. 1846, p. 23, 



pi. 1, f. I.B. M. Cat. Corb. p. 257, no. 49. 



The original type of this species "in Mr. Sowerby's collec- 

 tion" appears to be lost, and the description is too meagre to 

 separate it from its congeners : " C. t. ellipticd, ventricosd, 

 utrinque rotundatd, antice breviore ; dentibus parvis, obtusis." 

 Nevertheless, as it is further stated to be "outside white, with 

 an olivaceous epidermis ; inside whitish, varied with reddish 

 violet ;" and as it came from Mazatlan, it is fair to conclude 



