224 : 
PALAEONTOLOGY OF NEW-YORK. 
Rliynclionella semiplicata. 
Plate XXIX. Fig. la-o. 
Atrypa semiplicata : Conrad, Annual Report on the Palaeontology of New-York, 1841, p. 56. 
Shell (in the young state) compressed ovate, becoming more gibbous and 
subtriangular : 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 striae, crossed by con¬ 
centric undulations of growth. In the young state, the surface is free 
from plications, showing only fine concentric striae. 
This shell is frequently abundant in the lower part of the Pentamerus limestone, 
associated with Pentamerus galeatus and Strophodonta punctulifera. 
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 o. 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. 
Rliynclionella sequivalvis (n. s.). 
• i> ' 
Plate XXIX. Fig. 2 a - i; and Fig. 3 a, b, c. 
Shell ovate, somewhat compressed : sides sloping from 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. 
