LOWER HELDERBERG ROCKS. 
2G3 
LAMELLIBRAHCHIATA 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 Aviculae 
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 Ambonychise than 
Aviculm, 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 Aviculse 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. 
Telliiiomya imcleiformis ( n. s.). 
Plate XLIX. Fig. 1. 
Shell subelliptical or oval-ovoid, the anterior extremity regularly 
rounded, posterior extremity somewhat obtusely angular; umbones 
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. 
