264 TRANSACTIONS OF THE [mAR. 15, 



The anterior margin meeting the hinge-line at nearly a right angle, 

 the angle sometimes rounded; separated from the margin below 

 b}^ a rather abruptly rounded or angular byssal sinus, which is 

 more pronounced in the younger shells. Posterior wing less 

 comi^ressed, not sharpl}' separated from the umbonal slope, pos- 

 terior cardinal angle acute, posterior margin concave, joining 

 the postero-ventral margin by a gentle curve. 



Surface of the shell ornamented with simple radiating plica- 

 tions arranged in pairs, obsolete towards the beak and on the 

 younger specimens, though they would probably- be present on 

 the exterior of the shell. 



This species resembles A. coxanus M. k W.,* from the coal 

 measures, but differs from it in its larger size and greater con- 

 vexity. It differs from A. tcinchelli Meek,f of the Ohio 

 Waverly group, in its smaller size, greater convexit}' and rela- 

 tively longer hinge line. It differs from both these species 

 in having the posterior wing less flattened than the anterior and 

 much less sharpl}' separated from the umbonal slope. It re- 

 sembles A. monroensis, Worthen, | from the St. Louis group, but 

 is less oblique than that species. 



AvrCULOPECTEN ? Sp. 



PI. XX., fig. 13. 



Shell small, exclusive of the hinge-line subovate in outline, 

 oblique, moderately convex, longer than wide. Beak moderately 

 elevated above the hinge-line. 



Hinge-line a little shorter than the width of the shell below. 

 Anterior margin of the shell sinuate at the point where the an- 

 terior ear joins the bod\- of the shell, sharply rounded above the 

 sinuation and genth' arcuate below. Ventral margin regularly 

 rounded. Posterior margin arcuate below, nearlv straight 

 above, meeting the hinge-line at an obtuse angle. 



Wings subequal, not sharply compressed from the bod}- of the 

 shell. 



Surface ornamented with radiating plications, faintly shown 

 in the specimen as preserved, and obsolete towards the beak. 



Only a single specimen of this little species is present in the 

 collection. It maj- prove to be new, but for the present, in con- 

 sideration of the limited amount of material at hand, it is thought 

 best to leave it without a specific designation. 



*Geol. Surv., 111., Vol. 2, p. 32rt, PI, XXVI., figs. 6a-b. 



t Pal., Ohio, Vol. 2, p. 296, PI. XV., flg. oa-b. 



I Geol. Surv., 111., Vol. S, p. 114, PI. X'XII., figs. 8-9a. 



