PALAEONTOLOGY OF IOWA. 599 



width of the shell below ; cardinal extremities auriculate. 

 Ventral valve extremely ventricose, recurved, bringing the 

 beak opposite or below the centre of the valve, and nearer 

 the base than the width of the hinge-line ; marked by a 

 central longitudinal sinus, which is more or less strongly 

 defined, and reaches from near the beak to the base. Dorsal 

 valve moderately concave, and sometimes nearly flat in the 

 upper part and abruptly curved or geniculate in the middle, 

 the lower portion being rectangular to the upper ; sometimes 

 a slight elevation along the centre of the lower part. 



SURFACE of the ventral valve marked by regular rounded 

 radiating ribs, which bifurcate a few times, the bifurcations 

 occurring almost wholly above the centre of the shell ; 

 transversely marked by fine concentric striae, and some 

 strong wrinkles upon the upper part of the valve, and a few 

 inconspicuous undulations upon the middle of the shell. 

 Scattered, round, tubular spines occur on the middle and 

 lower part of the shell at the coalescing of the ribs, and 

 rarely a row near the base, with smaller ones sometimes at 

 the base of the ears and towards the hinge-line. 



Interior of the dorsal valve showing a bifurcate cardinal 

 process, each branch of which appears to be slightly bilobed 

 at the extremity, proceeding from a thickened interior car- 

 dinal margin, and connected with a short median ridge ; the 

 reniform vascular areas widely separated. 



In deference to numerous authorities, I refer this species to P.flemingi, indicating 

 at the same time its character as a distinct variety at least; and I have little doubt 

 that it will prove, when fully known, a distinct species. On the middle of the ventral 

 valve there are from 13 to 17 ribs in the space of ten millimetres, or 17 - 21 in the 

 space of half an inch, while there are respectively 10 - 15 and 14 -18 in the same 

 space on the margins. The vascular areas are proportionally much more widely 

 separated than represented in DE KONINCK'S figure of P.flemingi. 



Fig. 3 a. Cast of a dorsal valve where the shell is almost entirely exfoliated, leaving a 



mould of the exterior. 

 Fig. 3 6. Profile of the same, which shows only a part of the anterjor extension of the 



shell. 



Fig. 3 c. Ventral valve of the same species, 

 Fig. 3 d. Profile of the same. 



