162 



PALEONTOLOGY OF NEW YORK. 



Fig. \wi. 

 The brachulium otAtrypina 

 disparilis,U&\\. (c.) 



grooved, but the inner and outer divisions thus formed, are confluent at tlieir 



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 lamellsB 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, Leptocalia imbricata, Hall. Lower Helderberg group. 



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, Leptocoelia, Ccelospira, Trematospira, etc. Among these which 

 are evidently congeneric on the basis described, are Atrypa (jCculospira) 

 disparilis, Hall, of the Niagara group, Atrypina Clintoni, sp. nov., of the Clinton 

 fauna, Leptocalia 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 suhlepida, 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 fioin the lower Devonian of the Northei'n Urals, 

 liiniily furnished liy Prof. F. Schmidt, of St. Petersburg, show all the internal characters of Atrypina 

 imbricata. 



