LAMELLIBRANCHIATA. 323 



ities ; posterior end regularly rounded. Cardinal margin declining on both 

 sides of the beak. Anterior end abruptly rounded below. 



Valves ventricose. 



Beaks at about the anterior third, rounded, incurved. Umbonal region 

 very gibbous. Umbonal slope not defined, rounded. 



Surface marked by regular, sharp, concentric striae with a few varices of 

 growth. Interior unknown. 



A specimen has a length of 20 mm. and a height of 17 mm. 



This species somewhat resembles, in outline, N. bellistriata, but it is more 

 ventricose, the be.aks more erect and the umbonal slope is less defined. 



Formation and localities. In a conglomerate of the Chemung group, associated 

 with Ptychopteria Sao and Spirifera Vemeuili, east of Panama, N. Y., and at 

 Warren, Pa. 



NUCULA HOUGHTONI. 



PLATE XLV, FIGS. 29-31. 



NucuXa Houghtoni, Stbvbks. Am. Jour. Sci., 2d series, vol. 25, p. 262. 1858. 

 Nucvla Icwensis, Whitb and Wiiitfibld. Proc. Bo.st. Soc. Nat. Hist., vol. 8. 1862. 



" " Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 45, figs. 29-31. 1883. 



Shell of medium size, ovate, sub-trigonal ; length more than one-fourth greater 

 than the height ; basal margin broadly curving, abruptly rounding into both 

 extremities. Cardinal line sloping rapidly from the beaks to the anterior 

 and posterior extremities. Anterior end short, declining rapidly from the 

 beaks and abruptly rounded below. 



Valves gibbous, ventricose in the umbonal region. 



Beaks at about the anterior fourth, prominent, arching over the hinge-line. 

 Umbo prominent. Umbonal slope defined, rounded, extending from the beak 

 to the posterior extremity of the shell. 



Test strong, marked by very fine concentric striae which are scarcely fas- 

 ciculate on the body of the shell. 



There are about ten strong teeth posterior to the cartilage pit, and five 

 anterior ones. Muscular scars strongly impressed. 



