PANAMIC-PACIFIC PELECYPODA 235 



considerably between different specimens. The umbones high, full, with 

 the beaks adjacent and shghtly twisted in an anterior direction and some- 

 what in advance of the middle of the valve. Sculpture formed by concentric 

 incremental lines and sparser, little elevated, concentric threads. In life, the 

 shell appears to have been particularly liable to small accidents and injuries 

 producing uneven depressions and other abnormal irregularities of growth. 

 Interior of the valves white and smooth. 



Length 7.5 mm., height 6.5 mm., diameter 6.5 mm. (Dall). Capon, Peru. 



Length 9.2 mm., height 8 mm., diameter 2.7 mm. Venado Beach, Panama 

 Canal Zone. 



This is the commonest species of AUgena in the Panamic-Pacific region, 

 variable in shape. 



Range — Gulf of California to northern Peru. Panama Canal Zone: 

 Venado Beach. Peru: Tumbez. 



Allgena nncea Dall 



Aligena nucea Dall, 1913, Proc. U. S. Nat. Museum, vol. 45, No. 2002, p. 597 Gulf of 

 California. — Hertlein and Strong, 1947, Zoologica, vol. 31, pt. 4, p. 134. 



Shell small, white, rather solid, ovate, slightly inequilateral, moderately 

 inflated; surface rather rude, with irregular, rather coarse incremental lines; 

 beaks full, somewhat posterior, the anterior end of the shell shorter; interior 

 porcellaneous, the muscle scars unusually large, the pallial line irregular, 

 entire; hinge with a long, strong, narrow chondrophore, a small pustular 

 projection in front of it, as usual in the genus. Length of shell, 4.0, of an- 

 terior portion, \^S^ height, 3.0, diameter, 2.2 mm. 



This species recalls A. cokeri Dall of Peru, but wants the median radial 

 depression and has a proportionately stronger hinge. (Dall, 1913.) 



Range — Gulf of California south to Nicaragua. Mexico, Gulf of Cali- 

 fornia. Nicaragua, Corinto (Hertlein and Strong). 

 Genus OBOBITELLA Dall, 1900 



Type species by original designation, Montacuta jloridana Dall. Recent, 

 Florida. 



Shell small to large, subovate, inequilateral, the posterior side short 

 and rounded, the anterior side much longer. Hinge plate relatively narrow 

 but stout, with a single, pluglike tooth directly under the beak in each 

 valve, and behind it, an elongated, excavated or grooved, subumbonal 

 resilifer which extends across the hingeplate obliquely. Adductor scars sub- 

 equal, connected by an entire, ribbon-like pallial sinus. Surface smooth, more 

 often concentrically marked, sometimes earthy or chalky, the periostracal 

 coating thin, deciduous, plain or weakly rayed. 



Key to species of OrobiteUa 



I. Surface marked with microscopic or minute, divaricating lines or 

 striation (sagrination), best developed on the sides; destroyed on 

 worn specimens. 



A. Shell strongly convex. 



1. Shell large (length 13 mm.), the anterior side twice as long as the 

 posterior, its end obliquely rounded. 



0. stearnsi 



