222 
PALEONTOLOGY OF NEW YORK. 
In the pedicle-valve the delthyriiim is broadly triangular and is usually filled, 
partially or wholl}^, by the beak of the opposite valve. On the interior the 
dental lamellae make a strong spondylium which reaches almost to the bottom 
of the valve, being supported by a very low median septum extending nearly 
one-half the length of the shell. 
In the brachial valve there are two vertical crural plates not connected by a 
cardinal process. These are slightly convex on their inner surfaces and at 
their point of greatest convexity they unite with two longitudinal and gradually 
convergent lamellm, which form a spondylium narrower than that of the oppo¬ 
site valve, and supported by a very low median septum somewhat longer than 
that of the pedicle-valve. In a species from the Hudson River group, of 
Wilmington, Illinois, which has currently passed under the name of Camardla 
hemipUcata* this median septum is usually absent, the plates of the spondylium 
resting on the bottom of the valve, but in Atrypa hemiplicata and Pentamerus re- 
versus the small septum is always present. 
To such forms it is proposed to apply the term Parastrophia, assuming the 
Atrypa hemiplicata. Hall, as the typical species. 
This type of structure is continued upward into the faunas of the Niagara 
group, and in the dolomites of southern Wisconsin occur a number of interest¬ 
ing species, our knowledge of which has been derived from the elaborate col¬ 
lections made in that region by Thomas A. Greene, Esq., of Milwaukee. Here 
are at least three species which are new to science, all of them being preserved 
as most instructive internal casts. These are described in the Supplement to 
this Volume as Parastrophia Greenii, P. latiplicata and P. multiplicata, figures of 
all being given upon the accompanying plates. 
Among these shells there are no material variations except such as have 
already been noticed among the earlier species; for example, the spondylium 
of the more convex or brachial valve may be supported by a low median sep¬ 
tum for its entire length (P. Greenii), or for a portion of its length may rest upon 
* This form is much less extended than Atrypa hemiplicata, Hall; its plications are larger, sharper and 
fewer in number, and distinctly marginal. It is a shell quite different from the Trenton species, and may 
be termed Parastrophia divergens. 
