222 PALAEONTOLOGY OF NEW YORK. 



In the pedicle-valve the delthyrium is broadly triangular and is usually filled, 

 partially or wholly, 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 ne.arly 

 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 lamellae, 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 Camarella 

 hemiplicata* 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. latipUcata 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 ?iemipUcata, Hall ; its plications are larger, sharper and 

 fewer in number, and distinctly raai-ginal. It is a shell quite different from the Trenton species, and may 

 be termed Parastropltia divergens. 



