LAMELLIBRANCHIATA. 341 



specimen, is partially exposed, showing the characteristic crenulations of the 

 hinge. 



The specimen described has a length of 51 mm. and a height of 30 mm. 



The nearest relation of this form is with P. perplana, but it is a much larger 

 shell and its general form more distinctly elliptical. The surface markings on 

 the posterior part of the shell are i^recisely similar to that species, but the 

 specimen described is a cast, and an exact comparison cannot be drawn. 



Formation and locality. In the Hamilton group, at Patterson's creek, Va. 



PaLjEONEILO arata. 



plate l, fig. 23. 



PalcBoneilii arata. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. '>0, fig-. 23. 1883. 



Shell of medium size, sub-elliptical; length about twice the height; basal 

 margin convex in the anterior half, concave posteriorly, and rather abruptly 

 recurving. Posterior extremity somewhat prolonged and abruptly rounded. 

 Cardinal line nearly straight or slightly arcuate. Anterior end regularly 

 rounded. 



Valves moderately convex, gibbous in the umbonal region. 



Beaks between the anterior third and fourth, not prominent. Umbonal 

 slope convex, without defined ridge, merging in the general convexity before 

 reaching the posterior extremity. Post-cardinal slope concave, constricting 

 the margin of the shell. 



Surface marked by extremely fine concentric striae, which are often obscure 

 or obsolete, and by distant, strong, elevated, lamellose ridges. Interior 

 unknown. 



The specimen described has a length of 30 mm. and a height of 16.5 mm. 



This species somewhat resembles P. tmuistriata and P. fecunda, but there is 

 an entire absence of an umbonal ridge, or interruption in the curvature of the 

 posterior part of the shell, and the lamelliform ridges are stronger and more 



distant. 



44 



