New Species of Fossils from Hudson River Group. 



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more alate, the striae are finer, the umbones more convex, and 

 beaks more distinctly incurved and pointed. 



We have figured the hinge line of two specimens for the 

 purpose of showing that the cartilage grooves from the cardi- 

 nal teeth to the lateral teeth increase in number with the 

 size and age of the shell. Fig. 10 represents a thinner and 

 smaller shell than Fig. 9, and it has only two cartilage grooves, 

 while the specimen represented by Fig. 9 has four cartilage 

 grooves. This is a character noticed by one of the authors 

 in describing Anomalodonta gigantea, where the number of 

 lateral cartilage grooves varied in the specimens examined 

 from four to eighteen. It is probable that the number of 

 cartilage grooves increase with age in all the genera belong- 

 ing to the Ambonychiidae. 



This species occurs in the lower part of the Hudson River 

 Group, at Cincinnati. The specimens illustrated are from 

 the collections belonging to the authors. 



Pterixea cincinxatiensis, n. sp. 



Plate 1, Fig. 11, the co?ivex valve of a large specimen ; Fig. 12, 

 the opposite valve of the same specimen ; Fig. ij. the convex 

 valve of another specimen having the wings better preserved ; 

 Fig. 14, the convex valve of a smaller specimen; Fig. ij. 

 the opposite valve of the same ; Fig. 16, the convex valve of 

 a very small specimen ; Fig. 17 the opposite valve of the same. 



The first thing to which the attention should be directed 

 in the examination of this species, is the great diversity in 

 the size of the specimens collected. This we attribute to age, 

 the smaller specimens we regard as the young shells and the 

 larger ones as the mature forms. The surface markings of 

 the shells are the same, and the only differences noted in the 

 shape of the specimens, is that the wings become propor- 

 tionally more extended with the growth of the shells. 



Shell oblique, subrhomboidal in outline, with the basal 

 margin rounded and the posterior side contracted below the 

 point of the wing. Left valve depressed, convex in the 

 central part, and flattened toward the alations; right valve 



