PELECYPODA—PRIONODESMACEA. 



481 



convex. Surface only concentrically striated. Average length 2 

 inches ; height slightly over 1 inch. 



Judith River : Montana, Assiniboia. Laramie : Wyoming. 



LXXXVIII. Lyrodesma Conrad. 

 Moderately convex, equivalve, inequilateral. Beaks small, 

 anterior to the middle. Posterior umbonal slope usually promi- 

 nent. Surface concentrically striated. Hinge with 6 to 8 promi- 

 nent, subequal, transversely striated teeth radiating regularly 

 from the beak and placed on a thick plate. Posterior adductor 



Fig. 641. a-d, Lyrodesma major, a, right valve ; b, cardinal view, slightly en- 

 larged ; c, vertical section through valve at beak showing thickness of hinge plate and 

 reason why the beaks in internal molds are widely separated ; d, interior of right valve ; 

 e, f, L. acuminatum, right valve and its hinge, /"X 3- (After TJlrich. ) 



scar larger than anterior. Pallial line slightly sinuate posteriorly. 

 No ligament area present. Ordovicic and Siluric. 



315. L. acuminatum Ulrich. (Fig. 641, e,f.) Ordovicic. 

 Posterior cardinal slope with four or five radiating lines. 



Teeth 6, the central ones curved backward ; the anterior ones 

 short. 



Stones River : Kentucky. Black River : Minnesota. 



316. L. major Ulrich. (Fig. 64 t, a-d.) Ordovicic. 

 Shell much prolonged posteriorly. Posterior umbonal and 



cardinal slope marked with fine radiating lines. Hinge with 6 

 teeth. 



Cincinnati Group : Ohio, Minnesota. 



317. L. poststriatum (Emmons). Ordovicic. 

 Distinguished by its obliquely truncate posterior extremity and 



strongly striated posterior umbonal slope. 



Trenton-Lorraine : New York, Pennsylvania. 



