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 Atrypa reticularis, is 
unequaled by any other organism except that of the brachiopod Leptcena rhom¬ 
boid alls, 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 lamellse. 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 lamellse. 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. 
