THE LOWER SILURIAN ROCKS. 693 



Genus PHYSETOMYA, n. gen. 



Shell somewhat elongate, inflated anteriorly, tapering to a narrow or 

 _ acute extremity posteriorly; base arcuate. Beaks situated a little in front 

 of the middle, strongly incurved, not prominent though the umbones are 

 full. A narrow escutcheon and an obscurely defined luuule. Hinge ap- 

 parently edentulous; test very thin, muscular attachments not observed, 

 probably very faint. Surface with concentric striae and wrinkles of 

 growth, strongest anteriorly, and fine radiating lines. 



Type: P. acuminata, n. sp. 



This genus is believed to be closely related to Rhytimya, Ulrich, and 

 Ailorisma, King, differing from both in the subcentral position of the 

 beaks and tapering posterior end. 



Pysetomya acuminata, n. sp. 



Plate 49. Figs. 12-14. 



Shell small, transversely elongate, inflated and rounded in front, 

 tapering to an acute extremitj' behind; basal margin broadly arcuate on 

 the whole, somewhat straightened for a short distance in front of the 

 centre. Beaks strongly incurved, situated about 8 mm. behind the an- 

 terior extremity in a specimen 22 mm. long; umbones full, though a lit- 

 tle depressed in the middle by an obscure flattening of the surface that 

 extends vertically across the shell; posteriorly the umbo is drawn out 

 into a strongly rounded ridge which, however, is defined on the upper 

 side only by the abrupt descent to the hinge line. Escutcheon very nar- 

 row, extremit}^ about two-thirds the length of the hinge posterior to the 

 beaks; lunule obscurely defined, narrow. Surface markings consisting 

 of rather irregular concentric furrows, very indistinct on the posterior 

 cardinal slopes and a little the strongest on the anterior end. These are 

 crossed, at any rate on the posterior half of the ventral slope, by fine 

 radiating lines. 



The characters of this shell are so distinctive that I cannot see how 

 it might be confounded with any known Silurian lamellibranch. 



Formation and Locality: Middle beds of the Cincinnati group, at 

 Cincinnati, Ohio, where the specimen described was discovered by Dr. 

 S. A. Miller. 



