THE LOWER SILURIAN ROCKS. 691 



length is about three times the height. With this exception both his 

 description and figure are exact. The specimen here illustrated on plate 

 55 has suffered in nearly an opposite direction so that it is more elongate 

 than natural. But as at least nine-tenths of the specimens seen have 

 been compressed in a similar manner the figures now given may justly 

 be considered as highly characteristic of the shell as it is represented in 

 collections. 



Formation and Locality: Middle beds of the Cincinnati group at 

 several localities in the vicinit}' of Cincinnati, Ohio. 



Rhytimya convexa, n. sp. 



Plate 56. Figs. 2 and 3. 



Shell elongate, strongly convex, the outline narrowly subelliptical, 

 converging slightly toward each end from the middle; height equaling 

 nearly two-fifths of the length, thickness a little less than four-fifths ot 

 the height. Cardinal and basal margins gentle arcuate; anterior end curv- 

 ing obliquefy backward from the subangular extremity of the hinge line, 

 nearly vertical, however, in the upper half; posterior margin strongly 

 rounded and most prominent in the lower half, oblique and somewhat 

 straight in the upper. Beaks small, not very prominent, situated a little 

 less than one-fifth of the entire length of the shell from the anterior end; 

 posterior umbonal ridge subangular near the beaks, its convexity becom- 

 ing less as it recedes from them ; anterior umbonal ridge low, nearly ver- 

 tical ; sulcus rather shallow, not deep enough to cause a sinus in the basal 

 margin. Concentric surface markings unusually strong on the posterior 

 part of the valves, but anterior to the center they are precisely as in R. 

 oehana and other species of the genus. Postero-ventral fourth of valves 

 with series of large pustules arranged in a radial manner with respect 

 to the beaks. Pustules about one millimeter apart measuring from 

 center to center. 



Excepting that the concentric furrows are stronger on the posterior 

 half of the valves, the surface markings are about the same as in R. 

 mickleboroughi. Further comparisons with that species show that R. 

 convexa differs also in having a less prominent posterior umbonal ridge, 

 less attenuate ends, a shallower mesial sulcus, and a convex instead ot 

 sinuate ventral margin. In R. oehana the valves are a little higher, the 

 basal line gently sinuate, and the posterior margin more truncate. In R. 

 compressa the valves are less convex, the post-cardinal angle sharper, and 

 the mesial sulcus so little developed that it is practically wanting. 



Formation and Locality: Middle beds of the Cincinnati group, Cin- 

 cinnati, Ohio. It is found associated with R. mickleboroughi Whitfield, 

 sp., but is a much rarer shell. 



