•8 PALiEONTOLOGY OF NEW-YORK. 



This Bpecies is closely allied to the S. hemispherica, differing in the surface 

 marking, and to some degree, in the muscular impressions ; and it is impossible 

 to determine how far these modifications may extend, without a large series of 

 apecimens in better preservation than usually found. In the Hamilton group, the 

 species is subject to considerable variety of form and convexity, but is always 

 readily recognized among the other species of the group. In the collections there 

 IB a single example of a dorsal valve, which is moderately concave for a little more 

 than half its length, with some obscure concentric wrinkles ; beyond this, it is 

 deflected almost at right angles to the plane of the upper part of the shell. The 

 muscular impressions are shorter and wider than usual, and the cardinal process 

 is shorter and stronger. In comparison with the prevailing forms, this might have 

 been regarded as a distinct species. 



Geological formation and locality. This species occurs rarely in the Comife- 

 rous limestone in New-York, and is very common in the soft shales of the Hamil- 

 ton group in all the localities from Cayuga lake westward to Lake Erie. It is 

 apparently limited to the upper beds of the group, or to the Moscow shale, and 

 is comparatively rare below that horizon. At the present time, it is unknown to 

 me in any position higher than the Hamilton group. 



Stropliodonta perplaiia. 



PLATES XI, XII, XVII & XIX. 



SIrophomena perplana : Coxrad, Jonr. Ao»d. Nat. Sciences, Vol. viii, pa. 267, pi. 14, f. 11. 1842. 



Stropkomena phrittriata : Conhad, nt sup. p. 2o9. 



Strophomena delthyrU : ut sup. pa. 258, pi. 14, f. 19. 



Stropkomena crenUtria ; Hall, Report Fourth Greological Dist. New-York, p. 171, f. 4. 1848. 



Strophomena (Strophodonta) fragilis : IIali, Tenth Report on State Cabinet, p. 143. 1857. 



Strophodonta fragilU : Hall, Report Geological Survey of Iowa. pa. 496, pi. 8, f. 6a, 6c. 1858. 



Shell semielliptical ; the length varying from two-thirds to three-fourths 

 the width, which is from half an inch to two inches ; slightly concavo- 

 convex, and often nearly flat : hinge-line equalling or often a little 

 greater than the width of the shell below ; the extremities usually 

 somewhat salient, except in extremely old'shells. Margins of the shell 

 often a little contracted just below the hinge-extremities, making the 

 width less than below ; but the sides are frequently nearly straight 

 for half their length, and the front broadly rounded with the margin 

 attenuate. 



