3 l6 NEW YORK STATE MUSEUM 



the anterior slope the plications incline to multiplication by bifurcation, 

 while over the posterior slope they gradually grow obsolete, leaving the 

 extremity smooth. 



Dimensions. The single valve of this kind observed has a length of 

 35 mm and a hight of 17 mm. 



Habitat. Genesee province ; Naples subprovince. In the dark 

 Rhinestreet shale at Naples. 



modieixa Hall. 1884 

 Modiella sp. ? 



Plate 12, fig. 31 



A single specimen of this genus has been observed, representing a 

 small species like M. pygmaea Hall, with which it may prove to be iden- 

 tical. It presents the expanded posterior extremity, convex and broad post- 

 umbonal slope, oblique preumbonal depression and contracted convex 

 anterior extremity of that species, but does not show the fine radii in the 

 postumbonal slope. 



Habitat. Genesee province ; Naples subprovince. In the lower soft 

 shales at Naples. Modiella pygmaea is common in the Hamilton 

 shales of central and western New York. 



GASTROPODA 



pleukotomaria Def ranee. 1824 



The specific type of Pleurotomaria capi 1 1 ari a (Conrad) Hall, 

 which is a characteristic gastropod of the Hamilton fauna, is expressed by 

 shells with rather short spires and a surface ornament consisting of a few 

 (four or five) revolving ridges of unequal size crossed by retrally curved 

 concentric and finer ridges which form nodes at the points of intersection. 

 This style of sculpture is expressed in the Naples fauna with some variation 

 from that of P. capi 11 aria and so persistently that we judge it wise to 

 indicate these departures under distinct specific titles, rather than as muta- 

 tions of the earlier species, for, while they may be, and perhaps the weight 

 of evidence would indicate that these are genetically derived from P. 



