•94 PALEONTOLOGY OF NEW-YORK. 



Rliynclioiiella seniiplicata. 



Plate XXIX. Fio. 1 a - o. 

 JHrypa ttmiplicata : Conrad, Annual Report on the Palaeontology of New-York, 1841, p. 56. 



Shell ( in the young state) compressed ovate, becoming more gibbous and 

 sub triangular : valves nearly equal. Ventral valve slightly more gib- 

 bous, most prominent in the middle, and having towards the front a 

 more or less defined mesial sinus, in which there are from one to two 

 plications, while from one to three lateral plications occupy the space 

 on each side of the sinus : beak closely incurved over that of the 

 opposite valve. Dorsal valve depressed convex, having from two to four 

 rounded plications on each side of the medial fold, which is itself bifid 

 or trifid : plications strongly marked in front, and usually becoming 

 obsolete near the middle of the valve. 



Surface having traces of extremely fine radiating strlg3, crossed by con- 

 centric undulations of growth. In the young state, the surface is free 

 from plications, showing only fine concentric striaj. 

 This shell is frequently abundant in the lower part of the Pentamerus limestone, 



associated with Pentamerus galeatiis and Slrophodonta punclulifera. 



Fig. 1 a, b, c. Young individuals which are nearly smooth, or with faint undulations towards 



the front. 

 Fig. 1 d, e. A small individual having the front extremely plicated. 

 Fig. If-n. Illustrations of the prevailing forms, variety of plication, etc. 

 Fig. 1 0. Enlargement of the surface, showing the concentric and longitudinal striae. , 



Geological position and locality. In the lower pentamerus limestone of the Lower 

 Helderberg group : Helderberg mountains, Schoharie, Carlisle, etc. 



Rhynchonella scqulvalvis ( n. s.). 



Plate XXIX. Via. 2 a-i; and Fio. 3 a, 6, c 



Shell ovate, somewhat compressed : sides sloping fVom the beaks at a 

 little less than a right angle ; front semicircular : valves nearly equally 

 convex. Ventral valve having sometimes, towards the front, a broad 

 very faint depression or sinus : beak pointed and incurved. 



