50 MAZATLAN BIVALVES 
77. Donax navicuna, Hanl. 
Proc. Zool. Soc. 1845, p. 15.—C. B. Ad. Pan. Shells. p. 278. 
no. 456.—Rve. Conch. Icon. pl. 4, sp. 18. 
A eharming little species, both for its extreme beauty, and 
the distinctness of its characters. Shell singularly swollen, 
margin gibbous, scarcely shewing crenations beyond the edge 
at the anterior end: clothed with very glossy epidermis, be- 
neath which are obsolete strie. Ordinary colour greenish 
white, gradually assuming darker shades, often beaked with 
orange, generally more or less rayed, and ending in dark 
orange chesnut. Inside from white to fawn, ending in deep 
purple. Often (not always) with two streaks of purple within 
and without, bounding the lunular and posterior areas. It 
varies in the greater or less elongation, and in the ventral mar- 
gin which is sometimes incurved. One monster was found, 
shewing more prominent strie, long ‘44, lat.°9, alt. °29. 
Ordinary size ae Ne. Sear OOF 1 guar, eae 
Smallest Mawar ss <A, toy (ee 
Hab.—Gulf of Nicoyia, Cuming—Panama, HF. Jewitt, C. B- 
Adams.—Mazatlan ; not uncommon ; L’pool Col. 
Tablet 202 contains 5 specimens, most common variety, very 
pale.—203, 3 darker, tipped with orange.—204, 3 darker, 
slightly rayed.—205, 4 normal state, rather dark, indistinctly 
rayed.—206, 3 sp. rays more evident, tipped with orange.—207, 
3 sp. brownish orange.—208, 3 the same deeper.—209, 3 rich 
dark brown. 210, the monster.—In all, 28 specimens. 
Famity MACTRID/E. 
Genus MACTRA, Lin. 
78. Mactra EXoLeTa, Gray. 
Mag. Nat. Hist. 1837, p. 372.—Rve. Conch. Icon. pl. 4, sp. 16. 
This exquisitely beautiful species 1s recognized easily by 
the angle at the posterior side, and by the great prolongation of 
the flattened anterior margin, which rises into a sharp keel 
dividing the lunular portion. It differs from M. alata, Speng. 
(=M. carinata, Lam. teste Fve.?) in having no laminar keel 
on the posterior angle. The epidermis which is deciduous, 
but closely adherent when fresh, only shews strize of growth, 
and gradually fades away towards the umbos which are always 
