142 MAZATLAN BIVALVES 
Hanl. Descr. Cat. p. 155, pl. 18, f. 39.—B. M. Cat. D’ Orb. 
Moll. p. 81, no. 725.—C. B. Ad. Pan. Shells, p. 258, no. 416. 
=(Teste Krauss, Sudafr. Moll. p.16) Arca squamosa, Lam. 
An.s. Vert. vi. p. 474, no. 35.—Hanl. Deser. Cat. p. 158.— 
=(teste Desh.) Arca Domingensis, Lam. A. s. V. vi. p. 467, 
no. 16. (differing in colour.)—=(teste Desh.) Area clathrata, 
Defr. A. s. V. vi. p. 478, no. 6, (fossil.) 
Compare Byssoarca divaricata Sow. Proce. Zool. Soc. 1833, 
p- 18:—(Area d.) Rve. Conch. Ic. pl. 16, f. 108. (Annaa or 
Chain Is. attached to stones, Cuming ; W. Indies, B. M.) 
Comp. Byssoarea pusilla, Sow. Proc. Zool. Soc. 1833, p. 18 -— 
(Arca p.) B. WM. Cat. D’ Orb. Moll. p. 81, no. 717. (Iquiqui, 
Peru, on stones at low water, Cuming; Bolivia, Cobija, 
Peru, Arica, D’ Orbigny.) 
Comp. Arca donaciformis, Rve. Conch. Ic. pl. 16, f. 104:— 
Proc. Zool. Soc. 1844, p. 125, (Mozambique Channel, in 
Madrepore, Hankey.) 
This species varies considerably in the fineness or coarseness 
of the cancellated markings, in the projection or otherwise of 
the posterior rib, and in the shape, which often approximates 
B. solida, and is not unfrequently greatly appressed. Liga- 
ment very narrow, solid, only adhering at the posterior part, 
whenee it makes a slight turn round the umbos, as in B. illota. 
Hinge teeth rather few, slanting, on unequal lines slightly 
diverging. Muscular scars prominent, glossy white, as though 
ona single plate attached to the shell. The pedal chink is 
perceptible, but very small. These characters include speci- 
mens from the W. Indies, which are undoubtedly A. squamosa, 
Lam. also quoted by Krauss from Natal. If, as is probable, 
the species are identical, the Lamarckian name has priority. 
Another W. Indian species, probably A. umbonata, JA. s. v. 
p. 462, no. 5., differs in the fineness of the cancellations, and in 
the ligament which fills a central pit, as in B. solida. The 
A. pusilla of D’Orb. may be a dwarf var. of the Mazatlan shell, 
but the Cumingian type appears distinct. A specimen of the 
typical form measures long. ‘47, lat. °83, alt. °38. ‘ 
Hab.—Mazatlan, Beechey’s Voyage:—Do. rare, nestling in 
erevices of large shells; L’pool Col.—Sta. Elena, attached 
to stones, Cuiming.—Ecuador, Sta. Elena, D’ Orbigny.—Sta. 
Barbara, Col. Jewitt—Taboga; under stones near low water 
mark, very rare; C. B. Adams.—Also given from Ld. Hood’s 
Ts. and the Marquesas.—(A. squamosa.) ‘‘N. Hollande, a 
Vile King. Mus.” Lamarck.—Natal, Arauss—(A. Domin- 
