O AMERICAN JOURNAL 



Genus PROTHYRIS, Meek, 1869. 



Prothyris, Meek ; Proc. Ac. Nat. Sci. Phila., July, 1869, p. 172. 



Shell equivalve, very inequilateral, longitudinally oblong : 

 valves compressed or moderately convex ; nearly closed or a 

 little gaping behind, and more or less widely gaping in front,, 

 where the hiatus is increased in size by a nearly rectangular 

 notch in the margin, mainly below the middle ; beaks depressed 

 and very near the anterior end, with a small ridge usually ex- 

 tending from the anterior side of each to the corner of the ante- 

 rior marginal notch ; dorsal margin without escutcheon or lunule, 

 being erect and sharp behind the beaks ; surface merely marked 

 with striie of growth. Hinge and interior unknown. 



Type Prothyris elegans, Meek. 



In regard to the affinities of this genus it will of course be dif- 

 ficult to decide until its hinge and internal characters can be 

 known. The specimens being all casts of the exterior only, it 

 is necessarily from external characters alone that we can judge 

 of its probable relations. From its general form, however, and 

 open anterior end, I am inclined to think it related to the Sole- 

 nidce. Its form is not unlike that of some types of that group, 

 while its gaping anterior would seem to indicate that it had a 

 large foot, as we see in S'olen, Pharus, and other types of the 

 family. 



This genus seems to range through the whole of the Carbon- 

 iferous System. Until recently I had supposed the typical spe- 

 cies peculiar to the Upper Coal-measure beds at Nebraska City, 

 Nebraska ; but I have since seen it from different horizons in 

 the coal-measures of Illinois ; while another more convex species 

 has been discovered by Prof. Winchell in his Marshall group, at 

 the very base of the Carboniferous system in Michigan ; and 

 the same species has been discovered at that horizon during 

 the progress of the Ohio Geological Survey in that State. 



Prothyris elegans, Meek. PI. 1, fig. 3. 



Shell small, very thin, compressed, elongate-oblong, being 

 about three times as long as high ; basal and dorsal margins 

 straight and parallel, or nearly so ; posterior extremity obliquely 

 truncated, compressed and nearly closed ; anterior end more 

 gaping, but with its hiatus chiefly formed by the rectangular 

 marginal notch of each valve, which extends about half way up, 

 and has its lower margin rounding into the anterior basal edge, 

 and its upper into the little projection above ; beaks depressed 

 quite to the level of the dorsal margin, placed very near the an- 



