526 NORTH AMERICAN INDEX FOSSILS. 



Posterior extremity oblique, somewhat doubly truncate. Sinus 

 without distinct limitation, merely flattenening the shell above and 

 slightly depressing it below. 



Chemung Group : New York, Pennsylvania. 

 451. S. aeolus Hall. (Fig. 711, a,b.) Mississippi. 



Distinguished from 5". contractus by its larger size, more rounded 

 base, simply truncated posterior end, and lamellose growth lines. 



Waverly : Ohio. 



CXXII. Rhytimya Ulrich. 



Shell very thin, elongate, gaping slightly at both ends. Hinge 

 and basal margin subparallel. Beaks prominent. Mesial furrow 

 wide. Ligament external. Hinge apparently without teeth. 

 Muscle scars very faint. Surface covered with concentric folds, 



Fig. 712. Rhytimya radiata. (Ohio Survey, VII.) 

 Fig. 713. Rhytimya produda. (Ohio, VII.) 



strongest on anterior end, crossed on posterior half by radiating 

 rows of small granules. Ordovicic. 



452. R. radiata Ulrich. (Fig. 712.) Ordovicic. 

 Mesial furrow very slightly developed. 



Lower beds of Cincinnati Group of Ohio, Kentucky. 



453. R. producta Ulrich. (Fig. 713.) Ordovicic. 

 Surface crossed by about ten sharp concentric folds anterior to 



the beaks, becoming less sharp over the rest of the shell. Type 

 of genus. 



Middle beds of Cincinnati Group of Ohio, Kentucky. 



Fig. 714. Rhytimya mt'ckleboroughi. (Ohio, VII.) 



454. R. mickleboroughi Whitfield. (Fig. 714.) Ordovicic. 



Shell very elongate, anterior end acute. 

 Middle beds of Cincinnati Group of Ohio. 



