9S4 PALEONTOLOGY OF NEW-YORK, 



Spiiifera acuminata. 



PLATE XXXV : Fia. 24. 



Spiriftr aeuminatu* : Cokrad, See page 198 of this volume. 



DtlthyrU prora : Comrad, Jour. Acad. Nat. Sciences Fliiladelphia, Vol. viii, p. 268. 



" 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 Ilamilton group corresponding with Mr, Conrad's de- 

 scription, is one which I have identified with S. acuminata ; 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 specifio 

 distinction. 

 The localities in the Hamilton group have been cited on page 202. 



Spirifera subuiiibona. 



PLATE XXXII. 



OrthU tiibumbona : Haix. Tenth Report on the State Cabinet, p. 168. 1857. 

 Jmbocatia ttihumbcna : 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. 



Ventral 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. 



