150 
PALEONTOLOGY OF NEW YORK. 
the structure of the brachidium has been represented from silicified specimens. 
It may be remarked that in some of the preparations of this fossil the loop is 
situated somewhat nearer the middle of the primary coils. 
Many preparations have been made of the brachidium in shells of this species 
not only from New York, but also from the Trenton horizon of Rochester, Minn., 
Beloit, Wis., and Auburn, Mo. They have been found in various conditions of 
incrustation and replacement, but with a constancy of the characters described. 
Atrypa exigua has been playing a somewhat varied role in recent American 
literature. Sardeson has described it as a new species,* under the name 
Zygospira? aquila from the Trenton limestone at Minneapolis and other local¬ 
ities in Minnesota. Winchell and Schuchert have included it in a supposed 
primitive impunctate terebratuloid genus, Hallina, and have termed it Hallim 
NicoUeti.j Mr. Sardeson has been the first to give a figure of the internal 
structure of the shell {op. cit., fig. 18 ), the specimen represented having been 
cut in such a manner as to expose only the loop and that portion of primary 
lamellae lying behind its bases. The appearance of the brachidium is thus 
quite suggestive of some MAGELLANiA-like brachiopod. This writer, however, 
recognized the similarity of the brachidium to that of Zygospira, suggesting 
that in “ other sections there appear to be spiral coils anterior to the part shown 
in the figure, situated in the dorsal valve mainly, and with the apices together.” 
Messrs. Winchell and Schuchert have, from similar incomplete preparations, 
unfortunately misapprehended the shell. 
The value of the proposed genus Hallina can not, however, be estab¬ 
lished from the characters of this species only, as the type form specified by 
the authors is Hallina Saffordi, W. and S., from the Trenton or Glade limestone 
at Lebanon, Tenn. This is a small shell, oval in outline, and with biconvex 
valves which bear from fifteen to twenty subangular surface plications, begin¬ 
ning in the umbonal regions; it has, therefore, an altogether different exterior 
from Atrypa exigua, and is indeed not unlike an immature condition of the well 
known species, common in the Glade limestone and elsewhere at the Trenton 
* Ball. Minnesota Academy of Natural Sciences, vol. iii, No. 3, p. 335, pi. iv, ti{>:s. 15-18. 1892. 
t Amei’ican Geologist, vol. ix, p. 292. 1892. Geological Survey of Minnesota, vol. iii, pp. 471, 474, 
pi. xxxiv, figs. 59-62. 1893. 
