MAZATLAN BIVALVES 55 
? +(var.) Cytherea semifulva, Menke in Zeit. f. Mal. 1847, 
p. 190, no. 56. 
?=(var.) Cytherea gracilior, Sow. Thes. Conch. p. 615, pl. 128, 
f. 32.—(Trigona g.) Desh. B. M. Cat. Ven. p.53, no. 26. 
?+(var.) Cytherea Hindsii, Hanl. in Wood Suppl. pl. 15, f. 39: 
Proc. Zool. Soc. 1844, p.110.—Sow. Thes. Conch. p. 614, 
pl. 128, f. 27.—(Trigona H.) Desh. B. M. Cat. Ven. p. 53, 
nes Dery! verve 
?? + (var.) Cytherea intermedia, Sow. Thes. Conch. p. 615, no. 12, 
pl. 128. f. 35.—(Trigonai.) Desh. B. M. Cat. Ven. p. 49, no. 13. 
(Compare the Atlantic species T. mactroides, Born, Chemn. 
Dillw. [non Lam. nec Sow.| Desh. B. M. Cat. Ven. p. 51, 
no. 20: B. M. Cat. Sagra Cub. p. 40, no. 476:=C. corbicula, 
Lam. Hanl. Phil. Sow. [pars,] Gray, Menke | Zeit. f. M. 
1849, p. 40, non supra loe. cit.] Also T. Dillwyni, Desh. 
B. M. Cat. Ven. p. 49, no. 14:=C. mactroides, Sow. Thes. 
Conch. p. 615, pl. 128, f. 36 [non Lam.|—The W. African 
analogue is T. tripla Lin.: Desh. B. M. Cat. Ven. p. 52, 
no. 23:—Menke in Z.f. M. 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. Itis 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, 
* [have unfortunately distributed several specimens of the white var. of the 
form T. Hindsii, as another species ; but having then examined only about 100 
specimens, I thought the velvety epidermis was peculiar to that form and colour. 
T have since found it however (though it is generally cleaned off) on the typical 
T. radiata ; and after often repeated examinations of some 600 specimens, [ am 
compelled to unite them, 
