162 
PALEONTOLOGY OF NEW YORK. 
grooved, but the inner and outer divisions thus formed, are confluent at their 
outer extremities. The anterior face of the process is 
abrupt and vertical, its lower portion being continuous 
with the socket walls. In front of the cardinal process, 
but not supporting it, is a low median ridge, on either 
side of which are obscure muscular imprints. The 
brachial apparatus consists of introverted spirals whose 
bases lie against the lateral slopes of the pedicle-valve 
and whose apices are directed toward the center of the 
brachial valve. The ribbon is loosely coiled and makes 
but three or four volutions. The loop is situated posteriorly and constructed 
as in Atrypa, except that its lateral lamellae appear to be always united in an 
acute angle, which is directed inward. 
Muscular impression composed of large flabellate diductors, enclosing distinct 
adductor scars. 
Type, Leptoccdia imbricata, Hall. Lower Helderberg group. 
Fig. 153. 
The brachidium oiAtrypina 
disparilis,lla.\\. (c.) 
Observations. It has become necessary to establish a division for a number 
of little species whose structural characters have not heretofore been well known 
and which have, on that account, been referred indifferently to various genera, 
as Atrypa, Leptoccelia, Ccelospira, Trematospira, etc. Among these which 
are evidently congeneric on the basis described, are Atrypa (Calospira) 
disparilis, Hall, of the Niagara group, Atrypina Clintoni, sp. nov., of the Clinton 
fauna, Leptoccelia imbricata, Hall, of the Lower Helderberg group, and Atrypa 
Barrandii, Davidson, of the Wenlock limestone. This type of external and 
internal structure is continued upward into the lower Devonian where it is 
represented by the Terebratula sublepida, de Verneuil.* 
While atrypoid in external expression the shells differ from Atrypa, even in 
its broadest significance, in their uniformly small size, preponderating convexity 
of the pedicle-valve, few and very coarse plications usually crossed by fine 
* Preparations made from specimens of this species from the lower Devonian of the Northeim Urals, 
kindly furnished by Pi-of. F. Schmidt, of St. Petersburg, show all the internal characters of Atrypina 
imbricata. 
