PELECYPODA—PRIONODESMACEA. 379 



VIII. Ilionia Billings. 

 Anterior end of shell larger than posterior. A broad furrow 



extends from the umbos to the posterior basal margin. A large 



subovate muscular scar in the upper half of the posterior 

 extremity. No teeth. Siluric. 



13. I. galtensis Whiteaves. (Fig. 483.) Siluric. 



Fig. 483. Ilionia galtensis, left and right valves. (After Whiteaves. ) 



Umbos broad, subcentral. Surface concentrically striate. 

 Guelph : New York, Ontario. Cobleskill : New York. 



14. I. sinuata Hall. Siluric. 

 Surface marked by two furrows extending posteriorly from the 



beak, the one obliquely, the other vertically. 

 Cobleskill and Manlius of New York. 



IX. Orthodesma Hall and Whitfield. 



Transversely elongate, somewhat higher posteriorly than anteri- 

 orly. Valves thin, gaping at each end, usually marked by an 

 umbonal ridge, anterior to which is a broad depression. Surface 

 marked by more or less distinct concentric striae. Hinge plate 

 without teeth, thin and long. Ligament mostly external. Pallial 

 line simple. Ordovicic. 



15. 0. rectum Hall and Whitfield. (Fig. 484.) Ordovicic. 



■m0/0^- 



Fig. 484. Orthodesma rectum. (Pal. Ohio, II.) 



Cardinal and basal lines straight and converging forwards. 

 Posterior end obliquely truncate. 

 Cincinnati Group : Ohio, Indiana. 



