Pectcii.] PELECYPODA. 875 



Subgen. 2. CHLAMYS, Bolten, 1798. 



< 'hi, nn //*-, Bolten, Mus. Bolten. (2), 1798, 161. Type: Ostna ixillhuii, L. 

 MyocMamys, von Ihering, 1907. 



Valves moderately inflated, subequal, in general similar (except 

 in colour) ; sculpture of radial ribbing with or without divaricating 

 striation, with or without an imbricate surface layer ; frequently 

 spinose at the ridges ; ears often discrepant, the posterior smaller. 



Dist / ibut ion. World- wide. 



Fossil. Devonian to Tertiary. 



Sect. 1. CHLAMYS, s. str. 



Type : Oftlrea pallium, L. 



Ribs small and numerous, imbricate or spinose ; valves subequal, 

 similar, oblique, or with unequal ears, the posterior smaller ; divaricate 

 striation and imbricate surface layer usually present ; shell generally 

 solid and opaque ; byssal notch and ctenoliurn (set of spines, resembling 

 a short comb with curved teeth, on anterior margin of right valve at the 

 byssal sinus) present. 



KEY TO SPECIES. 



A. Number of ribs equal on both valves. 



a. 16-20 rounded ribs . . . . . . . . dithrous. 



b. About 40 unequal scaly ribs . . . . . . zdandice. 



c. About 80 subequal fine ribs . . . . . . radiat-us. 



B. Left valve with 11, right valve with 10 ribs, the latter un- 



equal . . . . . . . . . . . . imparvicostatus. 



2. Pecten dichrous, Suter, 1909. Plate 52, fig. 1. 



P. (Chlumys) dichrous, Suter, P. Mai. S., viii, 1909, 264, pi. 11, f. 31. 



Shell triangularly orbicular, moderately inflated, subequilateral. 

 somewhat inequivalve, with rather distant rounded radial ribs, valves 

 differently coloured. Ears very unequal, the anterior large, the pos- 

 terior very small. Beaks approximate, the prodissoconch small, oval, 

 smooth. Anterior and posterior ends similar, slightly concave above, 

 then forming a half -circle with the basal margin. Right valve slightly 

 more convex than the left, with a rather large triangular anterior 

 ear with 4 radial costse crossed by strong and close imbricating growth- 

 lines, a distinct byssal sinus below, and a row of teeth below it on 

 the anterior margin (ctenolium) ; the posterior ear very small, tri- 

 angular, with a few concentric riblets. Left valve with the anterior 

 ear also triangular, the anterior side straight or lightly sinuous, with 

 about 6 scaly radiate riblets^ the intercostal spaces with 1 or 2 fine 

 scaly lines. Sculpture consisting of subequidistant rounded radial 

 ribs, indenting the margins, the number varying from 16 to 20 on each 

 valve ; they are broadly or acutely rounded, sometimes ornamented 

 with scales, more prominent on the left valve ; towards the margins 

 the ribs have very often 1 or several grooves, and in the interstices 



