LOWER HELDERBERG ROCKS. 
This shell approaches very nearly, in some of its characters, the Rhynchonella 
ubrupta : 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 - c. Dorsal, ventral, and profile views of characteristic specimens. 
Fig. 1 f,g. A form more rotund than usual. 
Fig. 1 h. An individual with a strongly marked sinus and much elevated mesial lobe. 
Fig. 1 i. Profile view of a more gibbous form. 
Fig. 1 1c, l, in, n. More gibbous forms, which may probably be a variety of R. abrupla. 
Fig. 1 o. Enlargement of - the plications in front, showing the central impressed lines and 
arching strife. 
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. 
Sthynchonella altiplicata (n. s.). 
Plate XXXIII. Fig. 2 a - k . 
Shell sub trigonal, more or less gibbous. Ventral valve depressed convex : 
beak pointed, arclied 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. 
Surface 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 
