134 MAZATLAN BIVALVES 
181. Arca muLticostata, Sow. 
Proc. Zool. Soc. 1833, p. 21—Rve. Conch. Ic, pl. 4, f. 93. — 
Hanl. Ree. Shells, pl. 19, f. 12. : 
One very fine specimen of this shell was found by Mr. Archer. 
It differs from A. grandis in being much lighter, with more 
numerous ribs, and a squarer form. Long. 2°65, lat. 3616, 
alt. 2°36. 
Hab.—Gulf of Tehuantepec, 12 fm., Cuming—Mazatlan ; ex- 
tremely rare; L’pool Col. 
Tablet 628 contains a very small valve, ‘15 across, which 
may belong to this species.—629, a drawing of Mr. Archer’s 
specimen. 
182. Arca PLaprata, Sow. 
Proc. Zool. Soc. 1833, p. 21—Rve. Conch. Ic. pl. 1, f. 7.— 
Hanl. Descr. Cat. p. 160.—B. M. Cat. D’ Orb. Moil. p. 81, 
no. 720. 
Comp. A. labiosa, Sow. loc. cit.— Rve. Conch. Ic. pl. 10, f. 67.— 
Hanl. Deser. Cat. p. 159, pl. 19, f.3.—B. M. Cat. D’ Orb. 
Moll. p. 81, no. 720. 
Comp. A. incongrua, Say, Journ. Ac. Nat. Se. Phil. vol. i. 
p. 268.—Hanl. Deser. Cat. p. 159. 
The three species above quoted are very nearly allied. The 
two specimens found in the L’pool Col. by Messrs. Hibbert 
and Archer, before it fell into the dealer’s hands, are exactly 
like the specimens brought from Florida, by J. J. Audubon. 
Shell with distant beaks, and strong ligament filling up a large 
rhomboidal area. Shape subangular; tubercles absent from 
the angular part, also from the front of the smaller valve. 
Teeth rather broad. Zong. (from umbo) 1°23, lat.1°3, alt. 1°04. 
Hab.—Real Llejos and Tumbez, Peru, in sandy mud, 7 fm, 
Cuming.— Mazatlan ; extremely rare ; L’pool Col. 
Tablet 630 contains a specimen, presented by J. Hibbert, Esq. 
€ 
183. ARCA BIFRONS, 2. s. 
A. t. turgidd, subquadrata, tenui, maxiine inequivalvi, postice 
angulata, umbonibus haud distantibus ; alba, epidermide levi, 
olivaced indutd ; costis circiter xxx., in testd guniore omnibus 
tuberculosis ; in adulté solum viil.—x. anticis, reliquis levibus ; 
costis ventralibus in valvud minore parvis, rotundatis, 
