128 
PALEONTOLOGY OF NEW YORK. 
introverted lamella which forms an incomplete tube like that in Retzia, 
Hustedia, 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 flabellate 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. lU. l.i.ov oi ParazygaUrsuta^llaW. 
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 species has also been identified in the Corniferous limestone of Louisville, Ky., and elsewhere. 
t Named for the late Prof. Chester Dewey, of Rochester, N. Y. 
