«3«  PALEONTOLOGY OF NEW-YORK. 



Spirifeia angusta. 



PLATE XXXVIII A. 

 Spirtfer angtuta : Hali., Tenth Report on State Cabinet, p. 164. 



Shell very depressed pyramidal, extremely extended transversely ; car- 

 dinal extremities attenuate, acuminate : valves very unequally convex, 

 with surface finely plicate. 



Venteal valve proportionally very deep, forming nearly the entire thick- 

 ness of the shell : beak elevated and pointed, or slightly incurved at 

 the apex ; the cardinal margins sloping very regularly to the hinge- 

 extremities. Area extremely elevated and inclined forward ; the full 

 height equal to the length from the beak to the front of the valve ; 

 longitudinally and vertically striate, divided in the middle by a fora- 

 men which is but little more than half as wide as high, and distinctly 

 grooved on the sides for the reception of the deltidial plate. 



Dorsal valve very depressed-convex, with a narrow and low mesial fold 

 which is somewhat flattened at top : beak small but distinct, projecting 

 but little beyond the line of the hinge. Area extremely narrow, and 

 nearly rectangular to the area of the ventral valve. 



Surface marked by about twenty-four to twenty -eight fine simple rounded 

 plications on each side, few of which reach to the beak. The plications 

 are crossed by very fine concentric rugose striaB, which are slightly 

 arched upwards on- the plications and on the mesial fold. 

 Interior of the valves unknown. 



This species is usually readily distinguished by its extreme lateral extension, and 

 by the elevated and more or less receding area, which is usually nearly as high or 

 higher than the length of the ventral valve. There is frequently a slight incurvation 

 at the apex of the ventral valve, and sometimes the area is nearly vortical. In this 

 condition it approaches in general form to the young of S. medialis, which hiis 

 always a proportionally longer ventral valve. 



The specimens of this species hare usually a length of an inch or an inch and a quarter (as 

 in the accompanjing figure) ; while the largest individual observed is that figured on Plato 

 XXXVIII A, which has a width of more than two inches. 



