50 TERTIARY AND QUATERNARY PBCTENS OF CALIFORNIA. 



Subgenus Plagioctenium Dall, 1898. 



Type /'. circuluri'f Sowerby. 



Resembling- ^Equipecteit but without radial striation; the coneentiif sculpture 

 in looped lanielhv, the ribs strong, frequently smooth above; the su))margins 

 impressed below the subequal auricles; the valves well inflated with a tendency 

 to oblique growth in the adult. 



This group includes the following species of Pecten: AndevKoii'i^ (tiider^oiii 

 var. harkeridinis, cerrowiisls and its variety inendeiihuU! , circiduris and its variety 

 xqitivdcahiN, cooperi,. discus, deserti, ddridgei, ucdiienxix. pahloensis, pmicicostatus. 



Subgenus Pseudamusium H. and A. Adams, 1858. 



T}^pe P. e.rot/'eiis Chemnitz = P. pseudaniusiuni (Klein) Sowerby. 



Shells small, thin, more or less translucent; inner face of the disk without 

 lira^; sculpture d'screpant on the two valves, the right valve having the concentric 

 and the left valve the radial elements most pronoiuiced; valves usually flatfish 

 or compressed. 



The following Pectens are placed in this gi'oup: Paiu'dso/ii, loniplcoensis, pieck- 

 lidmi, pedroanns, nindo/jdii and its variety tilhniKnil-ensix, and wincouve/'ensis. 



Snl)gentis .\nnisiuni Bolten. 1798. 



Type P. plewrimrdcx Linne. 



Valves flattish, internall_y Urate, externallj- usually smooth or faintly striated, 

 both valves being similarly sculptured; the ctenolium absent and the byssal notch 

 obsolete. 



This group is represented by the single species P. lompocensh. 



Subgenus Propeanuisium (ircgorio, 1883. 



Type P. meqnlsoidj)tus Tiheri {-^ P. fenestratus Forbes). 



Right valve impressed about the distal margin, which is not fully calcifled. 

 partially concave, the sides partiallj' closed away from the ears; the internal line 

 pre.sent but shorter; the external sculpture chiefly concentric, while on the left 

 valve, if pi'esent, it is radial; the recent forms usually glassy or pale colored in 

 both valves. 



In this group are included the following Pectens: Alasl-ensh^ clallamensix, 

 intiiTrad'mtuH, riversi, stcmfordensts, and irmihindi. 



Subgenus Hinnites Defrance. 1821. 



Type H. corUzi. Defrance. 



Shell (up to advanced youth) a typical Chlamys, later becoming sessile and 

 irregular, in which stage the resilial pit is elongated and cardinal margin 

 develops an obscure area. 



This group is represented by the single species Peeten (jlganteus. 



