128 PALEONTOLOGY OF NEW YORK. 



introverted lamella which forms an incomplete tube like that in Retzia, 

 HosTEDiA, etc., but of no great extent. The teeth are as in Trematospira and 

 are supported by stout plates. The muscular area is short, rather well de- 

 fined, and is divided into a broad central adductor impression, along the lateral 

 margins of which lie two tlabellate diductor scars. 



The hinge-plate is very narrow, and is composed of two vertical supports 

 which have their origin on the downward umbonal slope of the interior. These 

 supports are widely separated at their bases and inclose the marginal dental 

 sockets; their anterior faces are vertical and their upper surfaces small and 

 sub-triangular. They do not unite with each other at any point, but each is 

 curved slightly back of the cardinal line, and on its ante-lateral angle supports 

 a crus. The loop is situated at about the center of the primary lamellae, bending 

 backward for a short distance and then forward at 

 an abrupt angle. Above this angulation its length 

 is about twice that below it. It terminates as in 

 Trematospira, in a short, sharp and simple hori- 

 zontal process, directed posteriorly. fig. in. L> op of Parazpgahiraula, um. 



The interesting combination of characters is best represented in the species 

 cited, Trematospira hirsuta, Hall, of the Hamilton group,* and with the exception 

 of the structure of the loop, the distinctive features were well illustrated on 

 Plate XLIII of Volume IV of the Palaeontology of New York. There is but 

 one other species which can properly be placed in the same association, namely, 

 the Waldheimia or Trematospira Deweyi,\ Hall, of the Lower Helderberg fauna. 

 This form is very similar to .Parazyga hirsuta in external characters, its surface 

 being finely plicated and with a median fold and sinus. Whether it was orig- 

 inally hirsute can not be decisively determined on account of the usual silicified 

 condition of the shells. The beak of the pedicle-valve is so closely incurved 

 that the foramen is almost or wholly obscured, and the deltidium has the 

 appearance of a concave excavation in the solid substance of the shell, having 

 thus almost precisely the structure occurring in Nucleospira. The hinge-plate 



» The 8|)ecie8 has also been identified in the Corniferous limestune of Louisville, Ky., and elsewhere, 

 t Name<l for the late Prof. Cubstbr Dkwbt, of Rochester, N. Y. 



