MAZATLAN BIVALVES 55 



? + (var.) Cytherea semifulva, Menke in Zeit. f. Mai. 1847, 



p. 190, no. 56. 

 ? = (var.) Cytherea gracilior, Sow. Thes. Conch, p. 615, pi. 128, 



f. 32. (Trigona g.) Desk. B. M. Cat. Yen. p. 53, no. 26. 

 ? + (var.) Cytherea Hindsii, Hani, in Wood Suppl. pi. 15, f. 39 : 

 Proc. Zool. Soc. 1844, p. 110. Sow. Thes. Conch, p. 614, 

 pi. 128, f. 27. (Trigona H.) Desk. B. M. Cat. Yen. p. 53, 

 no. 25. 



?? + (var.) Cytherea intermedia, Sow. TJies. Conch, p. 615, no. 12, 

 pi. 128. f. 35. (Trigona i.) Desk. B. M. Cat. Ven. p. 49, no. 13. 

 ( Compare the Atlantic species T. mactroides, Born, Chemn. 

 Dillw. [non Lam. nee Sow.] Desh. B. M. Cat. Ven. p. 51, 

 no. 20 : B. M. Cat. Sagra Cub. p. 40, no. 476 : = C. corbicula, 

 Lam. Hani. Phil. Sow. [pars,] Grray, Menlce \_Zeit.f. M. 

 1849, p. 40, non supra loc. cit.] Also T. Dillwyni, Desh. 

 B. M. Cat. Ven. p. 49, no. 14 : = C. mactroides, Sow. Thes. 

 Conch, p. 615, pi. 128, f. 36 [non Lam.'] The W. African 

 analogue is T. tripla Lin. : Desh. B. M. Cat. Ven. p. 52, 

 no. 23 : Menlce in Z.f. 7^1849, p. 40. The Chinese ana- 

 logue is T. ventricosa, Gray, B. M. Cat. Ven. p. 53, no. 24.) 

 This shell is never so tumid at the umbos [straight] as in the 

 African or [twisted] in the East Indian species : to the West 

 Indian T. mactroides however some forms bear so close an 

 analogy that Menke (and in part Sow.) may well be pardoned 

 for not discriminating them. Menke remarks that "its forms 

 are as various as its fatherland one should rather say mother- 

 sea." At Mazatlan however its forms and colours vary ex- 

 tremely in the very same mother-sea. It is generally pretty 

 equally swollen, but sometimes much flattened towards the 

 ventral part. Occasionally the whole shell is much compressed, 

 in which state it may possibly be T. intermedia, Sow. The 

 ventral line is sometimes well rounded, sometimes nearly 

 straight. Sometimes the posterior part is much produced, 

 occasionally however the anterior. It would be hard to give a 

 specific description that would include all the varieties and yet 

 exclude the W. Indian specimens : the general habit however 

 is so far distinct that I have not felt at liberty to follow Menke 

 in uniting them.* There is often, not always, an anterior gape, 



* I have unfortunately distributed several specimens of the white var. of the 

 form T. Hindsii, as another species j but having then examined only about 100 

 specimens, I thought the velvety epidermis was peculiar to that form and colour. 

 I have since founfit however (though it is generally cleaned off) on the typical 

 T. radiata ; and after often repeated examinations of some 600 specimens, I am 

 compelled to unite them. 



