TRENTON LIMESTONE. 101 



as the small Lingula does our Potsdam sandstone in the vicinity of Keeseville and other 

 places. 



Fig. 1 1 a. Dorsal valve, showing the central apex and lateral depression, which extends to the margin. 



Fig. 116. Ventral valve with marginal apex. 



Fig. lie. Lateral view of a perfect specimen. 



Fig. 1 1 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 Lovvville. 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. aqualis, and the transition from this to L. 

 curta is scarcely perceptible. On comparison, however, of a considerable jiumber 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 forpis. 



Presenting, as the Lingul.e 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 thosie 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, arfe clearly distinct species, and easily recognized 

 by the characters given. 



The Orbicul^ 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, an4 to have required the same kind of ocean bed or 

 sediment. 



