660 SYSTEMATIC PALEONTOLOGY 



centric; anterior and posterior dorsal areas usually differentiated; luiiule 

 frequently profound; escutcheon obsolete; ligament external, often deeply 

 sunken; normal dentition of right valve consisting of a simple anterior 

 cardinal, a bifid posterior cardinal and anterior and posterior laterals; 

 normal dentition of left valve consisting of a bifid anterior cardinal, a 

 simple posterior cardinal, and anterior and posterior lateral grooves; 

 laterals and less frequently the cardinals often obsolete; muscle impres- 

 sions strongly marked, the anterior elongate, the posterior oval ; inner 

 margins smooth or crenulated ; pallial line entire. 



The genus is abundantly represented in the Tertiaries, the Mesozoic, 

 and, if it be made to include the Prolucina of Ball, may be traced as far 

 back as the Silurian. The living species number more than one hundred, 

 and though they are most prolific in the tropics, they are present in the 

 temperate seas as well. 



PHACOIDES NOXONTOWXENSIS n. sp. 

 Plate XXXIX, Figs. 8, 9 



Description. Shell of moderate size, compressed, subcircular ; umbones 

 nearly central, acute, prosogyrate, not very prominent, dorsal slopes 

 gentle, the anterior a little less so than the posterior; lateral margins 

 broadly rounded; base line strongly and symmetrically arcuate; external 

 surface sculptured with acutely elevated concentric lamina?, regularly 

 spaced, probably about twenty-five in number, and between them very faint 

 secondary striations; ligament external, opisthodetic, lodged in a mar- 

 ginal groove elongated parallel to the dorsal margin; dentition obscure, 

 but two small diverging cardinals are distinctly present in each valve; 

 laterals apparently not developed; character of muscle scars and pallial 

 line not known. 



Dimensions. Altitude 26 mm., latitude 26mm., diameter 3.5 mm. 



This species is another of those Rancocas bivalves represented by 

 abundant fragments. The concentric sculpture is so well characterized, 

 however, that even a scrap showing the regularly arranged, acutely ele- 

 vated laminae is recognizable. As the species constitutes so important a 

 factor in the Rancocas fauna it does not seem wise to disregard it alto- 

 gether, even though the material is so ill-preserved. 



