%9t PALEONTOLOGY OF NEW-YORK. 



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Surface, in well preserved specimens, marked by fine close concentric 



striiB, with indistinct radiating striae ; th^ concentric striae sometimes 



crowded together in folds or ridges. 



A gntta-percha impression from a mould of apparently the same spe- 

 cies, and from which the natural cast, figures 27 and 28, was taken, is 

 distinctly radiatingly striate, as shown in figure 30. 



The cast of the ventral valve preserves evidence of moderately strong 

 teeth with dental plates reaching to the bottom of the rostral cavity, 

 where they are distinctly limited. The muscular area is only moderately 

 impressed, and the adductor imprints are cordiform : the surrounding 

 surface is papillose above and striate below. 



In the dorsal valve the muscular imprints are elongate, situated about 

 the centre of the valve, and separated by a median septum. The surface 

 of the cast is more or less papillose-striate. In some of the casts the 

 muscular imprints and septum are scarcely visible, but the latter, when 

 entirely preserved, has extended below the middle of the length of the 

 valve. 



This is a shorter and more rotund species than either of those described, and 

 maintains its proportions pretty uniformly. The smallest specimen measured has 

 a length of four lines and a half, and the original specimen figured (figs. 23-26, 

 pi. 47 ) has a length of eleven-sixteenths of an inch, with a Avidth Avhich is scarcely 

 less, and the depth is seven-sixteenths of an inch. A single ventral valve mea- 

 sures a little more than three-fourths of an inch in lenjrth. 



Figures 21-26 illustrate typical forms of this species. Figure 31 Js a cast of the dorsal- 

 valve ; and figures 27-29 are dorsal, ventral and cardinal views of a well marked specimen, 

 wkich, however, has not its characters very strongly defined on the dorsal side. 



The cast illustrating the interior characters, together with the gutta-percha 

 impression from the mould of the same, have so much the general aspect and 

 proportions of the A. polita that I have hesitated to separate them. All the 

 original specimens of A. polita are more or less exfoliated, though apparently 

 very nearly entire, but none of them give indications of continuous radiating stria?. 

 Although the striated surface is a departure from the prevailing surface characters 

 of Atiiyius, the cast does not present any features incompatible with a species of 

 thqt genus. 



Geological formation and localities. This species occurs in the Chemung group 

 at Jasper, Steuben county ; at Randolph and Albion, Cattaraugus county, and 

 other places in this group in Southwestern New-York. 



