170 PALEONTOLOGY OF NEW YORK. 



cate that the species at its latest appearance had undergone no variation in 

 form or surface-characters. The range through time, of Atnjpa reticularis, is 

 unequaled by any other organism except that of the brachiopod Leptana rhom- 

 hoidalis, Wilckens, and it far outranks that species in geographic distribution 

 and prolific individual development. 



Almost coincident in time with the appearance of Atrypa reticularis, in its 

 typical aspect, we find in the shales of the Niagara group shells which are per- 

 sistently small, with few and coarse plications, more or less distinct median 

 fold and sinus, and strong concentric lamellaB. These shells have been desig- 

 nated as Atrypa rugosa and A. nodostriata, Hall. The former is the smaller and 

 more extreme in the simplicity of its exterior. 



During the periods of the Lower Helderberg and Oriskany in New York, and 

 throughout the known extent of these faunas, such coarsely plicated shells 

 entirely disappeared from view, but returned in a depauperated condition in 

 the Corniferous limestone. In the Hamilton group they acquire a much larger 

 size and very gibbous form, the concentric lamellae being distant and strongly 

 developed. This is the shell known as Atrypa aspera, Schlotheim. At this 

 horizon the form mentioned is intimately associated with the typical, more 

 finely plicated A. reticularis, but abundant material affords no evidence of the 

 passage from one to the other. The coarsely plicated shell is continued into 

 the Chemung group, where, in New York, it presents a peculiar expression in 

 the much reduced number of its plications, and in the strong median elevation 

 of the brachial valve, which is not infrequently concave in the middle and 

 angular on the margins, these angulations becoming nodose from the elevation 

 of the strong concentric lamellae. In the calcareous sediments of the Chemung 

 group in the State of Iowa and other northwestern localities, the coarse-ribbed 

 shells also abound, though they possess a different expression than those of the 

 eastern Chemung fauna, having a very gibbous brachial valve without median 

 fold, and more conspicuous plications. They do not, however, approach even 

 remotely, the appearance of the typical A. reticularis, with which they are 

 associated. These shells have been designated by the term A. aspera, var. occi- 

 dentalis. Hall. 



