122 
MARINE SHELLS OF WEST COAST OF NORTH AMERICA 
marked carina, which in the case of the escutcheon is high, thin, and 
sharp, separated from another less acute radial keel by a wide, deep 
sulcus; on the disk near the middle are two other radials, evident but 
obscure, and another a short distance behind the lunular carina. The 
surface occasionally shows a faint dusting of microscopic granulation, 
which is usually abraded. Valves moderately convex, the interior polished, 
the hinge edentulous, nymphs slender and delicate, the ligament narrow 
and more or less visible externally, the margins of the valve indented by 
external ridges. Length, 20; height, 17.5; diameter, 15 mm. (Dali.) 
Type in U. S. N. M., No. 106870. Type locality, off Tillamook, 
Oregon. 
Range. Oregon to the Gulf of California. 
Thyasira tricarinata Dali, 1916. 
Proc. U. S. N. M., 52 :409. 
Shell chalky white, produced below, with pointed prosoccelous beaks 
over a deeply impressed ovate lunule bounded by a sharp carina; escutch¬ 
eon long, narrow, lanceolate, the valve margin rising as a sharp keel 
in the middle, the outer border very prominently keeled, outside of which 
is a similarly shaped excavated area also bordered by an angular keel; 
still outside of this there is a compressed area with no distinct anterior 
boundary except an obscure ray near the umbones; over this area the 
surface is concentrically striated, the rest of the disk being nearly smooth; 
hinge very feeble, ligament linear. Length, 15; height, 18; diameter, 
10 mm. (Dali.) 
Type in U. S. N. M., No. 209321. Type locality, Station 4425, off 
Santa Barbara Islands in 1100 fathoms. 
Range. Known only from the type locality. 
Subgenus Axinulus Verrill and Bush, 1898. 
Thyasira ferruginosus Forbes, 1844. 
Rept. Brit. Assoc., 1843, p. 192. Forbes and Hanley, Brit. Moll., 2; pi. 34, fig. 1. 
Minute, usually covered with a ferruginous coating, beneath which the 
surface is smooth; devoid of any fold. Only one denticle in either valve; 
no oblique cartilage groove on the hinge margin. (Forbes.) 
Type and type locality unknown to writer. 
Range. Aleutian Islands. Also Atlantic and Arctic. 
Genus AXINOPSIS G. O. Sars, 1878. 
Shell discoidal, tumid in the middle, compressed toward the margins; 
umbones slightly prominent; no external ligament; valves thin, pellucid, 
white, concentrically striate; cardinal tooth in the right valve obtusely 
