PAL-EONTOLOGY OF NEW-YORK. 



depressed along the centre; the beak projecting a little above the 



hinge-line, and with a narrow area gently incurved. 



The surface is marked by from eight to ten simple and somewhat 

 abruptly elevated plications on each side of the mesial fold and sinus : 

 these are crossed by strong lamellose imbricating lines of growth, which 

 give a varicose character to the surface, and where the shell is exfolia- 

 ted the plications are nodose. 



In some specimens, distinct fine radiating striae can be observed. 

 There is often a retral curving of the concentric strias in the centre of 

 the mesial sinus, and sometimes a slight elevation along that line. 



The original specimens of this species were obtained from a western locality ; 

 hut there are forms in Western New- York, which, though difForing slightly from 

 the originals, seem nevertheless referable to the same species. 



In comparing this with other generally similar forms, I remarked in my original 

 description that this species differs from S. eurutet'nes of Owen, in being smaller 

 and more transverse, with more angular plications and stronger concentric lines of 

 growth ; also, in the greater prominence of the two plications bounding the 

 sinus, as well as the abrupt and angular mesial fold. It differs from S. segmenta in 

 its fewer and more i-ugose plications, as well as in the more abrupt mesial sinus, 

 and iu the inclination of the area. 



The figures 1-4, Plate 31, are of tj-pical forms of this species, of which I have 

 more than a dozen individuals before me, all preserving the same characters with- 

 out material variation. 



Geological formation and locality. In limestone of the age of the Upper Hel- 

 derberg limestone at Charleston landing, Lidiana ; at Williamsville, etc., New- 

 York. 



Spirifera vailcosa, var. 



PLATE XXXI. 



Shell having the general characters of S. varicose, but larger and more 

 gibbous ; area higher, but similarly arcuate in the upper part. There 

 are ten or more plications on each side, which are rugose from strong 

 imbricating lamellae, and become somewhat nodose on partial or entire 

 exfoliation. Unfortunately only a few specimens of this variety have 

 been seen in New-York. 



