NATURAL SCIENCES OF PHILADELPHIA. 451 



obscure traces of small concentric wrinkles are indistinctly visible near the 

 beak and on the ears, particularly of the ventral valve. 



Length 0-59 inch ; breadth 0-60 inch ; convexity of ventral valve 0*34 inch. 



This shell might be mistaken for an imperfect specimen of P. fasciculatus of 

 McChesney, from which species it differs, however, in having finer striae, a 

 more deeply concave dorsal valve, and in being always destitute of a mesial 

 sinus in the ventral valve. When good specimens of these species can be 

 compared they may be distinguished at a glance, by the more extended 

 anterior, and the fasciculate character of the striae upon that part of the shell in 

 P. fasciculatus. 



Locality ani position. Chester, Illinois. Chester, Limestone of Lower 

 Carboniferous series. 



Productus scitdlus. Shell small, gibbous, wider than long, hinge line 

 rather more than equalling the breadth of the central part of the valves ; 

 anterior side rounded, or sometimes a little flattened; sides rounding to the 

 front, and somewhat contracted near the ears. Ventral valve gibbous, rather 

 strongly arched, moderately pro luced, ani presenting scarcely any indications 

 of a sinus ; ears triangular, convex, or somewhat vaulted, and separated from 

 the swell of the umbo and the prominent visceral region, by a rather 

 distinct rounded depression ; beak convex, incurved and apparently extend- 

 ing a little beyond the hinge line. Surface marked by numerous fine, 

 regular, radiating striae, about seven of which may be counted in the space of 

 one-tenth of an inch ; crossing these there are on the visceral region small, 

 irregular concentric wrinkles ; a few scattering bases of spines are also 

 sometimes seen on the anterior slope (in casts), where two or three of the 

 striae become more prominent than the others. (Dorsal valve unknown). 



Length, from beak to anterior slope, O3o; do., from beak to anterior 

 margin, measuring over the curve of the ventral valve, 0-45 inch. 



Locality and position. Alton, Illinois. St. Louis Limestone, of Lower 

 Carboniferous series. 



Genus RHYNCHONELLA, Fischer. 



Rhynchonella subtrigona. Shell rather above medium size, trigonal 

 subglobose, wider than long, truncated in front, and on each postero-lateral 

 slope ; anterior margins of the valves sharply and deeply serrated. Ventral 

 valve nearly flat, or arching a little from the umbo along the middle towards 

 the front, on each side of which the anterodateral mxrgins are at first elevated, 

 then very abruptly deflected toward the other valve ; front curving down 

 nearly at right-angles to the plane of the valve, and extended so as to fill a 

 broad, deep, rather roundel sinus in the front of the other valve; postero- 

 lateral margins very abruptly deflected downwards ; beak small, rather 

 pointed, incurved, and extended somewhat beyond that of the other valve : 

 mesial sinus broad, shallow, undefined, and not extending more than half-way 

 back from the front. Ventral valve gibbous, elevated in the middle near the 

 anterior side, thence sloping abruptly, with a moderately convex outline to 

 the beak; antero-lateral and lateral margins curving strongly to meet those 

 of the opposite valve ; mesial fold not well defined. Surface of each valve 

 ornamented by from eighteen to twenty-four rather rounded plications, about, 

 fou r or five of which occupy the mesial sinus and fold ; fine obscure, concentric 

 striae are also seen on well-preserved specimens. 



Length (of a melium-sized rather gibbous specimen), - 90 inch; breadth 

 0-98 inch; convexity 0*91 inch. 



Locality and position. Keokuk Limestone, Warsaw, Illinois. 



Genus ATHYRIS, McCoy.=SPIRIGERA, D'Orbigny. 



Athyeis parvirostra. Shell of medium size, subquadrate, moderately 

 gibbous, length and breadth about equal, sometimes a little wider than long ; 



I860.] 



