292 
PALAEONTOLOGY OF NEW-YORK. 
The figures 1-7 of Plate xlyii illustrate the general features of this species. 
Geological formation and locality. In the Hamilton group at Delphi, New York. 
A single specimen from the Chemung group is undistinguishable from those of 
the Hamilton group. 
AtJiyris angelica. 
PLATE XLYII. 
JLthyris angelica : Hall, Fourteenth Report on the State Cabinet, p, 99. 1861. 
Shell ovoid, gibbous, transverse or elongate ; proportions of length and 
width variable, the prevailing form longer than wide ; deeply sinuate, 
with the beak very prominent ; liinge-line short. 
Ventral valve gibbous, most convex above the middle ; mesial sinus 
usually extending nearly to the beak, becoming abruptly and deeply 
depressed below the middle, and much expanded towards the front of 
the shell ; lateral portions of the valve gibbous, and abruptly curving 
towards the margins : beak much elevated and curved over the umbo 
of the dorsal valve. 
Dorsal valve much shorter than the opposite, gibbous, transverse or as 
long as wide ; the mesial fold usually not conspicuous above the lower 
third of the valve : in the upper part the surface is a little more gibbous, 
and the striae along this part are more straight or a little curved back¬ 
wards, indicating the form during the successive stages of growth. 
Surface marked by regular equidistant imbricating lamellae or lamelli- 
form striae, the edges of which in perfect specimens are projecting 
and slightly crenulate. These lamellae are marked by short inter¬ 
rupted radiating striae, as shown in figure 20. 
The interior of the ventral valve shows a semicircular perforation at 
the apex, opening on the lower side to a triangular fissure. The teeth are 
strong, and continued in plates to the base of the rostral cavity. 
The adductor imprints are strongly marked just below the rostral cavity, 
and the divaricator imprints occupy an ovate space below and on each 
side. Surface of cast, adjacent to the muscular imprints, strongly 
papillose. 
