TEENTON LIMESTONE. 101 
as the small Lingula does our Potsdam sandstone in the vicinity of Keeseville and other 
places. 
Fig. 11 a. Dorsal valve, showing the central apex and lateral depression, which extends to the margin. 
Fig. 11 b. Ventral valve with marginal apex. 
Fig. 11 c. Lateral view of a perfect specimen. 
Fig. 11 d. Ventral valve, marked with strong concentric lamellae ; apex marginal. 
Position and locality. This species is not uncommon at Middleville, Herkimer county, 
and perfect specimens are found at Lowville. It occurs also at Watertown, Turin and 
Trenton Falls. {State Collection.) 
Of the forms of Lingula above presented, 1, 2, 3 and 6, are more nearly allied to each 
other, while all the others are clearly distinguishable. If the upper part of L. riciniformis 
were extended, and the shell a little depressed, it would correspond with L. attenuata ; 
while if the proportions were altered in the opposite direction, or the beak depressed and 
sides expanded, it would correspond with L. cequalis , and the transition from this to L. 
curia is scarcely perceptible. On comparison, however, of a considerable number of speci¬ 
mens, I am induced to believe that it is desirable to separate them ; calling attention to 
these differences, and thus inducing a comparison of a larger number of specimens. We 
may thus, hereafter, be able to point to some more decided characters, or show the 
gradations of the same species through all these forms. 
Presenting, as the Lingula: for the most part do, but few prominent features, and being 
seen but rarely, it is not easy to distinguish them, or to point out characters which mark 
specific distinctions, as many of those relied upon may be only indicative of different ages 
of the shells, or of different periods of their growth, and may even be dependent on other 
circumstances unknown to us. 
The other forms, beyond those noticed, are clearly distinct species, and easily recognized 
by the characters given. 
The Orbicula: are usually associated with the Lingula:, in the fine-grained compact 
limestone ; the Conularia is also an almost constant associate of these shells, and rarely 
found in other situations. These shells, so unlike in structure and habit, appear to have 
flourished under similar circumstances, and to have required the same kind of ocean bed or 
sediment. 
