LOWER HELDERBERG ROCKS. S63 



LAMELLIBRANCHIATA OF THE LOWER HELDERBERG GROUP. 



The physical conditions attending the deposition of the strata of the 

 Lower Helderberg group were more favorable to the existence of this class 

 of molluscs, than were those of the Clinton and Niagara groups in the 

 State of New- York : there are, nevertheless, comparatively few species 

 known at the present time. In the Tentaculite limestone the Aviculaa 

 are very rare, or not occurring at all; while nearly all the species of the 

 Shaly limestone are of this genus, and several occur in the Lower Pen- 

 tamerus limestone. The aviculoid forms in the lower beds are gibbous, 

 essentially equivalved shells, more nearly resembling Ambonychiae than 

 AviculaB, but, so far as can be observed, wanting the external ligamental 

 area which is so characteristic of some of the Lower Silurian species 

 included in that genus. There are likewise a few Cypricardia-like forms, 

 approaching in character to some species of the Clinton and Niagara 

 groups, but which differ in their external aspect from most of those, in 

 the presence of concentric ridges which are more or less strongly marked 

 in the different species. With the exception of some of the Aviculae and 

 one or two other forms, all the species are very rare ; there being, in 

 all the collections made, no more than one or two individuals of each 

 species. 



Telliiioiiiya nucleiforiiiis ( n. s.). 



Plate XLIX. Fig. 1. 



Shell subelliptical or oval -ovoid, the anterior extremity regularly 

 rounded, posterior extremity somewhat obtusely angular ; umbonea 

 prominent, finely striated concentrically ; muscular impressions, in the 

 cast, very prominent ; crenulations on the hinge-line about twenty or 

 twenty-one visible in the cast. 



