88 PALyEO^'TOLOGY OF NEW JERSEY. 



over which the striations do not extend. Anterior end sharply rounded 

 and the basal line round and full. Posterior extremity oblique, extended 

 somewhat below and rounding into the basal line. Posterior umbonal ridge 

 rounded but quite distinct. Surface covered entirely with fine, slightly raised 

 thread-like striae, which are somewhat alternating in size on the postero- 

 cardinal slo})e. Hinge plate moderately wide and distinctlv arched on the 

 inner margin, the line of teeth more distinctly arcuate. Teeth numerous, 

 narrow, and diverging outward from beneath the beak, and gradually 

 increasing in length to near the outer ones. Muscular impressions propor- 

 tionally large and distinct, the posterior one having a slightly raised line on 

 its anterior margin, but only seen in the larger specimens. 



The species closely resembles, in its general form, Idonearca Shumardi 

 of the Upper Missouri and Black Hills Cretaceous, but differs in being 

 radiately striated, and in the form of the teeth. Among the Cretaceous 

 forms of New Jersey it I'esembles in form Ncmoarca cretacea Con. most 

 nearly, but differs entirely in the form of the surface markings. These 

 close resemblances show how entirely unsatisfactor}' are these close generic 

 di'. isions. The specimen which I have figured on Plate xii, figs. 1 1 and 12, 

 appears to be the same with that used by Mr. Conrad for generic figures in 

 1872, and I have made the figures as accurately as it is possible to measure 

 the specimen. 



Formation and locality. — In the micaceous clays beneath the Lower 

 Green Marls at Haddonfield, New Jersey. From the collection of the Acad- 

 emy of Natural Sciences of Philadelphia. 



Genus TEIGONAECA Conrad. 1862. 



(Proc. A. N. S., 1863, p. 289, and Am. Jour. Concb., Vol. Ill, i>. 9.) 



Tiigonarca cvineiformis. 



Plate XII, Figs. 17 and Iri. 



Trigonarca euneiformis Conrad. Am. Jour. Conch., Vol. V, p. 98, PI. IX, Fig. 1. 



Shell quite small, not exceeding half an inch in extreme length in any 

 of the examples yet observed. Trapezoidal in outline and quite ventricose. 

 Hinge but little more than one-half the length of the shell, and the area 

 ver}' narrow. Beaks small, slightly incurved Anterior end of the shell 



