LAMELLIBRANCHIATA. 479 



Surface marked by fine concentric striae, which are in some parts crowded 

 and fasciculate on the body of the shell ; marked by distinct undulations 

 upon the anterior end and post-cardinal slope. 



Four specimens measure respectively 56, 57, 70 and 74 mm. in length, and 

 18, 19, 22 and 25 mm. in height at the posterior extremity. 



This form is one of the typical species of the genus described by Mr. Conead, 

 and in the absence of any special knowledge of 0. pholadis it must be regarded 

 as the type. It is a common form and has been widely recognized. 



Formation and localities. In the Hamilton group, at Bear's gulf and Fulton- 

 ham, Schoharie county ; at several places in Otsego and Onondaga coimties and 

 on the shores of Seneca, Cayuga and Canandaigua lakes ; and rarely in the 

 Genesee valley. 



Orthonota cabinata. 



IT-ATE l-XXVUI, I'IGS. 34, 35. 



Orthonota carinata, Conrad. Geol. Siii-v. N. Y., Ann. Rep., p. T)!. 1841. 



" " Hall. Prelim. Notice Lamellibranchiata, 2, p. 87. 1870. 



" " " " Pal. N. Y., vol. V, pt. 1. Plates and Explanations: PI. 78, 



figs. 34, 35. 1883. 



Shell small, elongate, cylindrical ; length more than five times the height. 

 Basal margin inflected, essentially straight, gradually curving anteriorly and 

 abruptly recurved posteriorly. Posterior extremity doubly emarginate below 

 and obliquely truncate above. Cardinal line long and straight, extending 

 the greater part of the length of the shell. Anterior end short, narrowly 

 rounded. 



Valves regularly convex in the anterior half. 



Beaks sub-anterior, low, rounded and scarcely defined. Umbonal slope 

 angular, the ridge extending to the post-inferior extremity and limited on the 

 lower side by a narrow groove. Post-cardinal slope longitudinally marked, 

 above the middle, by a strong angular ridge with a smaller longitudinal 

 ridge midway to the umbonal angle. These ridges give the posterior slope 

 of the shell a strongly plicate appearance and produce abrupt sinuosities in 



