266 NEW YORK STATE MUSEUM 



for the most part simple but of very unequal size, and are separated by 

 sharp grooves. Low concentric ridges cross the surface without modifying 

 these radii. This style of ornament is most unusual in all aviculoid shells, 

 and we do not find its parallel elsewhere. This species is therefore readily 

 distinguished, however much it may have been subjected to disfigurement. 



Dimensions. Average examples measure 4.4 and 6 mm in hight ; 3.8 

 and 6 mm in length. 



Habitat. Genesee province ; Naples subprovince. We have found 



this shell only at two localities east of the Genesee river; Pogues hill, 



Dansville, where it occurs in some abundance, and near Union Corners, 



Livingston co. Chautauqua subprovince : Williams's specimens were 



obtained from the shales about Attica, Genesee co. It also occurs at 



Fox's point, Lake Erie, and Big Sister creek, Angola. 



Note. While discussing shells of this character, attention may be 

 directed to a species occurring occasionally in the bituminous Marcellus 

 shales, which is so closely similar to Pteroch. fragilis that it is readily 

 and has been confounded with it. This resemblance is remarkable ; 

 the shells are of about the same dimensions and shape, both have tenuous, 

 glabrous shells, essentially the same contour and rounded postcardinal 

 margin. The difference between them lies in the development in Ptero- 

 chaenia of a deep byssal furrow, broad extended byssal flanges with a wide 

 hiatus between them. In the Marcellus species, which, for sake of a name, 

 we may designate as Posidonia marcellensis, the 

 byssal flange maintains its aviculoid character as a long 

 auricle of the dimension of the Pterochaenia flange, 

 delimited by a very low depression from the body of the 

 shell. It would be very difficult to find specific features, 

 so far as they can be ascertained by comparison with the 

 figures and descriptions given by Freeh, 1 in which this 

 shell differs from Posidonia hians Waldschmidt, 

 Fi K . 12 Posidonia un less perhaps in the latter the auricle is more sharply 

 ralve.xs 6 ' defined. Such shells serve to indicate the relationship of 



Pterochaenia to the aviculoids. Posidonia marcel- 

 lensis occurs in the lower black shale beds at Marcellus, Onondaga co. 



l Op.cit. p. 72, pi. 14, f.g. 13. 



