PELECYPODA—PRIONODESMACEA. 



5ii 



very thin, probably perforate. Surface of both valves marked 



with distant, rounded, radiating ribs. Cretacic. 



405. P. scabra (Morton). (Fig. 685.) Cretacic. 



FlG. 685. Paranomia scabra. (After Weller, Pal. N. J., IV.) 



Ribs produced at irregular intervals into hollow spines. 

 Ripleyan of New Jersey (Merchantville-Navesink), Tennessee 

 and Gulf region. 



CVIII. Modiolopsis Hall. 



Shell thin, obliquely elongate, very inequilateral, with small 

 anterior and large posterior end. Beaks nearly at anterior ends. 

 Valves crossed by an oblique depression extending backward from 

 the anterior portion of" the umbo. Teeth absent, occasionally 

 represented by an obscure thickening and corresponding depres- 

 sion. Muscle impressions two ; anterior deep, and posterior large 

 and faint. Ligament chiefly external, deep seated. 



Differs from Modiomorpha in the absence of teeth. Ordovicic 

 and Siluric. 



406. M. faba (Conrad). Ordovicic. 

 Very small, with prominent umbonal ridge and sinus. 

 Trenton : New York, New Jersey, Wisconsin. 



407. M. mytiloides Hall. (Fig. 686, b.) Ordovicic. 



Fig. 686. a, Modiolopsis concentrica ; b, M. mytiloides. (Minn. Surv. ) 



Subcylindrical, marked by fine concentric lines interspersed with 

 stronger wrinkles. Junction of posterior margin with hinge line 

 almost angular. 



