334 
PALAEONTOLOGY OF NEW-YORK. 
Spirifera acuminata. 
PLATE XXXV : Fig. 24. 
Spiri/er acuminatus : Conrad, See page 198 of this volume. 
Delthyris prora : Conrad, Jour. Acad. Nat. Sciences Philadelphia, Vol.viii, p. 263. 
“ Upper valve trigonal, profoundly ventricose; ribs double, not very 
“ prominent, eighteen or twenty in number; mesial fold profoundly 
“ prominent and acutely angular; hinge-line shorter than the width 
“ of the shell : length one inch and five-eighths; width two and a 
“ half inches.” 
The only species in the Hamilton group corresponding with Mr. Conrad’s de¬ 
scription, is one which I have identified with 8. acuminata y the dorsal valve of 
this is shown in figure 24, Plate xxxv. The ribs are more conspicuously bifurca¬ 
ting than in those from the limestone, and the muscular impression is larger; but 
both these features are variable, and therefore cannot be relied upon for specific 
distinction. 
The localities in the Hamilton group have been cited on page 202. 
Spirifera snbumbona. 
PLATE XXXII. 
Orthis subumbona Hall. Tenth Report on the State Cabinet, p. 168. 1857. 
Jlmboccelia subumbona : Id. Thirteenth Report on the same, p. 71. 1860. 
Shell small, more or less gibbous or ventricose; cardinal extremities 
rounded : surface smooth, or finely striated concentrically. 
Yentral valve ventricose in the middle, regularly curving towards the 
basal and lateral margins : umbo much elevated above the opposite 
valve, and beak abruptly incurved over the high area, which has its 
lateral margins rounded or rarely defined, and sloping towards but not 
reaching the cardinal extremities; more or less arcuate, and the eleva¬ 
tion apparently variable. The foramen is higher than wide, and open 
to the apex in all the specimens observed. There is usually a narrow 
but not always distinct mesial sinus, reaching from the apex to the 
base of the valve, where it becomes wider but without defined limits. 
