1897.] NEW YORK ACADEMY OF SCIENCES. 261 



riorly at an angle of about 27^. Posterior margin incomplete in 

 all the speeimens examined, but judging from the lines of 

 growth, meeting the hinge-line at a right angle or less than a 

 right angle. 



Surface ornamented with simple, indistinct radiating plica- 

 tions and with even less distinct concentric wrinkles of growth. 

 The radiating plications are least distinct towards the ventral 

 border, while the concentric markings grow more distinct in 

 this region. 



This species is similar to P. maxviUensis, Whitfield,* but may 

 be distinguished by the well-defined longitudinal depression just 

 below the hinge-line, which Professor Whitfield definitely states 

 is not present in his species, and by the much fainter radiating 

 and concentric plications. The Arkansas shell is also larger 

 than P. maxviUensis if Whitfield's figure may be taken as repre- 

 senting the ordinary size ; its maximum length, as determined 

 by projecting the edges of the broadest of the incomplete speci- 

 mens, is about five inches. It diflfers from P. fiexicostata, Mc- 

 Coy, f with which Whitfield compares P. maxviUensis, in hav- 

 ing less distinct plications. P. missouriensis, Swallow, J is 

 apparently an allied form, but it has never been figured, and the 

 original description is so meagre that no comparison is practi- 

 cable without authentic specimens. 



Pteronites hopkinsi sp. nov. 

 Pi. XX., fig. 10. 



Shell, exclusive of the posterior wing, obliquely sub-ovate. 

 Yer}' convex anteriorl}-, somewhat depressed posteriorly. Pos- 

 terior wing large, moderately depressed, not sharply separated 

 from the body of the shell. Valves sub-equal, the left one 

 slightly more convex and with a slightly larger beak. Beaks 

 terminal. 



The hinge-line straight, nearh' equal to the greatest posterior 

 extension of the shell below. The antero- ventral margin, judging 

 from the lines of growth, forms a semi-oval curve, becoming 

 more abrupt posteriorly where it bends up to meet the posterior 

 margin. Posterior margin slighlty sinuate below its junction 

 with the cardinal line, meeting the antero-ventral margin in a 

 sub-semi-circular curve. 



Surface ornamented with radial plications which are strongest 



*Ann. N. Y. Acad. Sci., Vol. 5, p. 586, PI. li, fig. .5, and Geol. Surv. Ohio, Vol. 7, p. 

 474, PI. 10, fig. 5. 



t British Pal. Foss., p. 499, PL 3E. figs. 11-1:3. 

 t Trans. St. Louis Acad. Set., Vol. 2, p 97. 



