MO'LLUSCA. 413 



All the examples of Area quindecemradiata in both the recent 

 collections of the Survey and in the older collections, whose hori- 

 zon is definitely known, are from either the Hornerstown marl 

 or the Vincentown limesand, and it can be stated with almost 

 absolute certainty that the species is limited to these horizons, 

 although the records with some of the older specimens would 

 seem to indicate that it may possibly have been found rarely in 

 the Navesink. 



Formation and locality. — Vincentown limesand, near Hurff- 

 ville (171). 



Geographic distribution. — New Jersey. 



Genus Nemoarca Conrad. 



Nemoarca cretacea Conrad. 

 , Plate XXX., Pigs. 25-26. 



1869. Nemoarca cretacea Con., Am. Jour. Conch., vol. 5, p. 97, 



pi. 9, fig. 21. 

 1886. Nemoarca cretacea Whitf., Pal. N. J., vol. i (Monog. 



U. S. G. S., vol. 9), p. 86, pi. 12, figs. 8-10. 

 1905. Nemoarca cretacea Johns., Proc. Acad. Nat. Sci. Phil. 



(1905), p. 9. 

 Description. — "Shell small, seldom attaining more than half 

 an inch in extreme length, trapezoidal in form, the transverse 

 •diameter being nearly once and a half the height. Valves very 

 ventricose, with large, strongly inflated, prominent beaks, situ- 

 ated nearly opposite the middle of the length. Hinge-line straight 

 and low; area narrow, the length a little less than the greatest 

 length of the body of the shell. Hinge-plate narrow, marked by 

 about 12 short, oblique teeth which diverge from the center on 

 each side, and twoi or three transverse teeth nearly parallel to the 

 hinge-line at the posterior end. Muscular imprints too. faint to 

 be observed on well-preserved casts of the interior. No' internal 

 rib bordering the posterior scar. Surface marked by from four to 

 six fine radiating ribs on the posterior slope, and 24 toi 26 on the 

 body of the shell and anterior end. Strongest on the posterior 

 part of the body of the shell and gradually decreasing in size 



