EARLY DEV0XIC HISTORY OF NEW YORK AND EASTERN NORTH AMERICA 47 



Schizophoria multistriata Hall 



Plate 10, figures 12-18 



Or this multistriata Hall. Palaeontology of New York. 1859. 3:176, pi. 15, 



fig. 2 a-t 

 Schizophoria multistriata Hall & Clarke, op. cit. v. 8. pt 1, p. 212 



The representatives of this species at Dalhousie are quite well defined 

 but attain a uniformly and notably larger size than in the Helderberg of 



New York. It is a common shell, while in the New Scotland beds of New 

 York it is rare. 



Horizon. Nos. 1, 9. 



Rhipidomella hybridoides Clarke 



Piute 10, figures 19-28 



Rhipidomella hybridoides Clarke. N. Y. State Mas. Bui. 107. 1907. p. 282 



But for the extravagant size this shell attains at full growth it would 

 be quite impracticable to distinguish it from American forms of Sowerby's 

 well known Upper Siluric O r t h i s h y b r i d a. In its immature stages it 

 is essentially that shell ; at full growth its characters have changed by 

 progression and indicate thereby a Postsiluric age. 



Horizon. No. 2. 



Rhipidomella numus Clarke 



Plate 11, figures 1-12 • 



Rhipidomella numus Clarke. N. Y. State Mus. Bui. 107. 1907. p. 283 



A shell directly comparable to R. (Orthis) obi at a Hall of the 

 Helderberg fauna, agreeing therewith in form and contour of valves though 

 perhaps never attaining the size of that species. It differs therefrom : 

 (1) in the slightly greater length of hinge, but principally (2) in the very 

 much coarser and sparser plication of the surface. In R. oblata the 

 radial striae are fine and crowded ; in a typical example I find about 70 at a 

 distance of 10 mm from the beak and at the anterior margin in a shell 32 

 mm long, 190. In the largest example of R. nu mus, 24 mm long, there 

 are 40 at 10 mm from the beak, 106 at the margin. Thus there are practi- 

 cally two striae in R . oblata to every one in R . n u m u s ; those of the 

 latter angular, multiplying rapidly. When compared with the rarer Helder- 

 berg species R. eminens, its plication is still much coarser, its hinge 

 not so lone and it lacks the elevated ventral beak of that shell. 



The species is quite abundant. 



Horizon. No. 9. 



