3'26 PALEONTOLOGY OF NEW YORK. 



Atrypa hystrix. 



PLATE LIII A. 

 Mrypa hystrix : Uall, Report of Fourth Geological District, p. 272, f. 2; p. 271. 1843. 



Shell suboval, ovate or subcircular, usually flattened: surface acutely 

 costate. 



The shells of this species are too imperfect and obscure to admit of 

 detailed description. The valves are usually much flattened, and the im- 

 prints are left in shaly sandstone or shale from which the shell has been 

 removed. The surface is marked by a few distant angular or subangular 

 ribs, Which are sometimes bifurcated, and are crossed by strong lamellose 

 strijB. In the perfect shell, long spines proceed from the lamellae at the 

 crossing of the ribs ; and when the shell is exfoliated, the ribs are nodose 

 from the spine-bases. 



The collections furnish a single gibbous specimen of the dorsal valve, one 

 inch and a quarter long by about an inch and a half wide, with a depth 

 of about three-fourths of an inch, and other specimens of smaller size. 



The young specimens of A. spinosa approach this form, as shown in figure 8 of 

 Plate Lin a, and it is possible that this may be an extreme variety of that species. 

 The specimens, figures 16 and 17, occur in localities 'vrhere forms like figure 18 are found, but there 

 are no palpable gradations; and among a hundred individuals like figure 18, and figure 1 of 

 page 325, there is no deviation which can be regarded as an approach to figures 16 and 17. 



Geological formation and localities. This species is known only in the middle 

 and higher part of the Chemung group, and has been principally found to the 

 south and southwest of Bath in Steuben county. 



^% From collections recently made in Iowa, we learn that in all localities the 

 distinction between Atrypa reticularis and the associated species is more strongly 

 marked than in the collections from New York, and there is nowhere any indica- 

 tion of gradation from the one to the other. In the higher beds of the series in 

 Northern Central Iowa, which may be of the age of the Portage or Chemung 

 formation of New York, the species identified with A. reticularis is more finely 

 cotitate, while thg other form approaches more nearly to the A. hystrix of our rocks 

 having a few coarse plications with spines ; these appendages, however, are 

 rarely preserved. 



