144 AXEL A. OLSSON 



in young shells may not be developed at all. Inner layer white, brilliantly 

 pearly, divided longitudinally in the middle into two sections by the deep 

 sulcus which forms the keel on the surface. 



This species has an elongated, trigonal shell ornamented with eight 

 rows of large, tubular spines. Adult specimens have a heavily thickened, 

 coarse shell which may reach a length of nearly two feet. 



Range — Lower California to Panama. Panama: Taboga Island; Pearl 

 Islands. 



Genus ATRDfA Gray, 1842 



Type species by subsequent designation, Gray, 1847, Pinna nigra 

 Dillwyn. 



Shell fan-shaped, wider posteriorly, like Pinna but the nacreous layer 

 is not divided by a longitudinal sulcus. Arranged in two subgenera on 

 basis of the position of the posterior adductor muscle scar. 



Key to species of Atrina 



I. Posterior adductor scar large, not confined wholly to the nacreous area 

 but its edge extending slightly beyond it. 



Subgenus Atrina, s.s. 



1. Ventral side not straight but contracted and deeply impressed near 

 the anterior end, the posterior side widely expanded. Rows of scales 

 numerous, coarse, convex, or vaulted. 



A. tuberculosa 

 II. Posterior adductor scar lying wholly within the nacreous area. 



Subgenus Serv atrina 



2. Ventral side nearly straight, the shell having a fairly uniform tapered 

 form from the beak to the posterior margin. Rows of scales relatively 

 few in number (7 to 10), the dorsal ones large and tubular. Color 

 usually purplish amber. 



A. maura 

 Subgenus ATRINA, s.s. 



On the inside of the valve, the posterior adductor muscle scar lies 

 close against the hind edge of the nacreous area or protrudes slightly beyond 

 it. 

 Atrina (Atrina) tuberculosa (Sowerby) 



Pinna tuberculosa Sowerby, 1835, Proc. Zool. Soc. London, p. 84. — ^Reeve, 1858, Conch. 

 Icon., vol. 11, Pinna, pi. 25, fig. 48. 



Shell broadly trigonal or fan-shaped, the anterior end short, con- 

 tracted, the posterior side high and widely expanded. Surface sculptured 

 with rows of coarse spines (13 to 16), foliaceous and vaulted. The color 

 dark-brown to nearly black. Specimen figured by Reeve is nearly 200 mm. 

 long. 



This species is the Eastern Pacific analogue of the Atlantic A. rigida 

 (Solander). Its range is more northerly than A. maura, being most plenti- 

 ful in the Gulf of California. 



Subgenus SERTATRINA Iredale, 1939 

 Type species by original designation. Pinna assimilis Reeve. 

 Externally like Atrina, s.s. but internally with the posterior adductor 

 muscle scar enclosed within the nacreous area and which generally 



