LOWER HELDERBERG ROCKS. 231 



This shell approaches very nearly, in some of its characters, the Rhynchonella 



abrupta : there are, indeed, some forms which it is difficult to distinguish. In the 



well-characterized specimens of this species, it differs from that one in its smaller 



and more numerous plications, and in being proportionally less ventricose, as well 



as in the narrower and deeper sinus of the ventral valve. The general aspect of the 



shells is quite distinctive. 



Fig. 1 a - e. Dorsal, ventral, and profile views of cliaracteristic specimens. 



Fig. 1/, g- A form more rotund than usual. 



Fig. 1 h. An individual with a strongly marked sinus and much elevated mesial lobe. 



Fig. 1 J. Profile view of a more gibbous form. 



Fig. 1 k, I, m, n. More gibbous forms, which may probably be a variety of JR. abrupta. 



Fig. 1 o. Enlargement of the plications in front, showing the central impressed lines and 

 arching striae. 



Fig. 1 p. Cast of the ventral valve, showing the lobed muscular impression. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Albany and Schoharie counties. 



JEMiynchonella altiplicata ( n. s.). 



Plate XXXIII. ¥ia.2a-k. 



Shell subtrigonal, more or less gibbous. Ventral valve depressed convex : 

 beak pointed, arched or nearly straight. Dorsal valve the larger, most 

 elevated in the middle, declining with a curved outline towards the 

 beak and margins : beak incurved ; foramen triangular, extending to 

 the apex of the beak. 



SuBFACE marked by from ten to about nineteen simple, strongly elevated, 

 sharply angular plications on each valve ; two to four of which are 

 elevated on the dorsal valve into a more or less distinct mesial promi- 

 nence extending nearly to the beak; and from one to three depressed 

 on the middle of the ventral valve into a distinct sinus, which widens 

 regularly and somewhat rapidly from near the beak to the front, where 

 it is prolonged into a short projection, filling a corresponding sinus in 

 the front of the opposite valve : shell traversed by fine concentric 

 lines of growth. 

 Along the lateral slopes of the cardinal margin, on each side of the beaks, there 



is generally an oval space of greater or less extent, not plicated : this, although 



