WAVERLY GROUP SPECIES. 301 



swell of the umbonal slopes by a faint undefined sulcus extending from 

 immediately behind each beak, obliquely backward and downward to 

 the truncated edges of the posterior ends of the valves. Surface orna- 

 mented by usually well-defined concentric ridges and furrows, that are 

 small and very regular on the umbones and strongest anteriorly, but 

 generally become obsolete on the posterior dorsal region; crossing these 

 a small, very obscure sulcus may often be seen extending from each beak 

 nearly directly downward to the base. 



Length of a well-developed adult specimen, 1.35 inches ; height of do., 

 0.81 inch ; convexity, 0.71 inch. 



Although always presenting peculiarities of general physiognomy by 

 which it can be readily recognized, this shell varies much in form, as 

 well as in the size and regularity of its concentric ridges and furrows. 

 The furrow extending down each valve from the beaks, is generally ob- 

 solete, or so faintly defined as scarcely to attract attention, though it is 

 quite distinct on some specimens. On some individuals, like that rep- 

 resented by our figure 5a, the concentric ridges and furrows are very 

 strongly defined, and comparatively large, while on others they are 

 smaller, as seen in figure 5c, and on still others they fade away and become 

 so irregular as not to be readily distinguished, on the lower and posterior 

 parts of the valves, from the marks of growth. 



As may be seen by the synonymy, this shell was originally referred by 

 Dr. Shumard to the genus Allorisma, King, and more recently it has been 

 referred by others to Grammysia, DeVerneuil, with which latter it cer- 

 tainly seems to be rather closely connected through other species. Still 

 it appears to me to be quite as nearly related to Prof. McCoy's genus 

 Sedgwickia, as originally founded by him in 1844, on his S. attenuata, S. 

 bellata, S. corrugata, etc., though it is widely distinct from the original 

 typical forms of the group {Lepfodomus) to which he refers such shells in 

 1855. 



Locality and position : The specimens we have figured are from the Waverly sand- 

 stone of the Lower Carboniferous, at Medina, Ohio. It also occurs at Cuyahoga Falls, 

 and various other localities at the same horizon in Ohio, as weli as in the yellow 

 arenaceous beds at Burlington, Iowa ; also at Hannibal, Missouri, where the speci- 

 mens described by Dr. Shumard were obtained. It is likewise found at this horizon 

 in Illinois, and is said to occur in the Chemung group (Devonian) in New York and 

 Pennsylvania. 



