136 MAZATLAN BIVALVES 
Tablet 632 contains 2 specimens, squarish form.—633, 5 do. 
fine flattened growth.—634, 5 do. normal growth.—635, 3 do. 
swollen, produced.—636, 3 do. swollen, oval.—637, 4 sp. shewing 
the inside, one normal, another with the anterior teeth irregu- 
lar, the third with the teeth nearly obsolete, the fourth do. 
quite obsolete, apparently from disease. 
185. ARCA REVERSA, Gray. 
Sow. in Proc. Zool. Soe. 1833, p. 20.—Mill. Syn. Test. Viv. 
p. 180.— Rve. Conch. Ic. pl. 1, f.5.— B. M. Cat. D’ Orb. Moll. 
p. 81, no. 722.—C. B. Ad. Pan. Shells, p. 261, no. 421. 
Arca hemicardium, Koch in Phil. Abbild. pl. 1, f. 1. 1843. 
This aberrant species is known at once by the truncation of 
the anterior portion. The ligament is cut off at right angles 
between the umbos, (which are not distant) and is solid, occu- 
pying the posterior area. The posterior line of teeth begins 
from the umbo; the anterior begins from a point nearer the 
middle, lying within the other, and is very short. There is a 
slightly developed ridge bounding the anterior adductor, as in 
Cucullea. Long, 1°7, lat. 2°15, alt. 1°5. 
Hab.—Tumbez, Peru, in soft mud, 7 fm., Cuming, D’ Orbig- 
ny.—-Panama, extremely rare, C. B. Adams.—Mazatlan ; 2 
fine specimens only were found by Mr. Archer; L’pool Col. 
Tablet 638 contains one specimen. 
186. ARcA P BREVIFRONS, Sow. 
Proc. Zool. Soc. 1833, p. 22.— Rve. Conch. Ic. pl. 1, f. 6. 
In consequence of the internal characters not being given, 
the species cannot be determined with accuracy, but it fits the 
description as far as it goes. Epidermis as in A. multicostata, 
very finely striated along the radiating ribs, coarsely rugose 
between. Umbos approximate; ligament very long and nar- 
row. Inside white, with a subumbonal stain as in A. emar- 
ginata. Posterior hinge teeth in a long shghtly curved line: 
anterior somewhat twisted, in a short line at a decided angle. 
The teeth resemble A. reversa. Long. °59, lat. °48, alt. °33. 
Hab.—Tumbez, Peru; in soft mud, 7 fm.; Cuming.—Mazatlan ; 
1 sp. on Murex nigritus, with A. emarginata; D’pool Col. 
Tablet 639 contains the specimen, 
