OF CONCHOLOGY. v 



terior end, and so compressed as to be very inconspicuous ; valves 

 without any post-umbonal ridge or prominence, but showing a 

 faint marginal impression along near the cardinal border, and 

 another below this, extending obliquely to a point just above the 

 middle of the truncated posterior edge ; ridge extending down 

 in front of the beaks to the corner of the notch, almost linear, 

 and narrowing upward ; while, in some specimens, there are ap- 

 pearances of two other much smaller linear ridges, or little im- 

 pressions,* farther back, diverging from each beak. Surface 

 stride of growth rather sharply defined and regular on the ante- 

 rior ventral region, but nearly obsolete elsewhere. 



Length of the largest specimen seen 1 inch ; height 0-33 inch ; 

 convexity about 0-07. 



The typical specimens of this species were collected by Dr. 

 Hayden and his assistants during his geological survey of Ne- 

 braska, in the Upper Coal-measures at Nebraska City. I have 

 fully described and illustrated the genus and species in Dr. 

 Hayden's Report, but as it may not be published for some con- 

 siderable time, and this is the type of a new and interesting 

 genus, it has been thought desirable to give a notice of it here 

 with an outline figure. 



Specifically this shell will be readily distinguished from the 

 only other known species, already alluded to, from a much lower 

 horizon (Proihyris Meeki, -Winchell, MS.), by its much more 

 compressed valves without any distinct posterior umbonal pro- 

 minence, straighter dorsal outline, more depressed beaks, and 

 less widely gaping and rather more distinctly notched anterior 

 margin. 



This shell is rather common in the clay beds at Nebraska City, 

 Nebraska, and, as already intimated, it occurs less numerously 

 at different horizons in the Coal-measures of Illinois. 



Martesia ? Roessleri, Meek. PI. 1, fig. 4, 4a. 



Shell cuneate-ovate, being very gibbous anteriorly, and nar- 

 rowed and compressed posteriorly ; posterior extremity narrowly 

 rounded ; anterior side very short, moderately gaping, the hiatus 

 having apparantly become gradually narrower with the growth 

 of the valves ; beaks rather prominent, gibbous, incurved and 

 located near the anterior extremity ; basal margin semiovate in 

 outline, being most convex a little in advance of the middle, 



* The anterior one of these appears to be an extremely slender, linear, 

 external ridge, while the other has more the character and appearance 

 of an impression made by an internal ridge through the very thin shell. 

 This is doubtless a generic character. 



