TRENTON LIMESTONE. 101 



as the small Lingula docs our Potsdam sandstone in the vicinity of Kccseville and other 

 places. 



Fig. 1 1 a. Dorsal valvo, showing tho central apfi.x and lateral depression, which extends to the margin. 



Fig. 1 1 b. Ventral valve wilh marginal apex. 



Fig. 1 1 c. 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 LowviUe. 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. ricinijormis 

 were exterided, and the shell a litile depressed, it would correspond wilh L. attenuata ; 

 while if the proportions were altered in the opposite direction, or the beak depressed and 

 sides expanded, it would correspond with L. aqnalis, and the transition from this to L. 

 curta 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. Vt'e 

 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 LiNOULiE 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 Orbicul.e are usually associated with the Lingul.t:, in the fine-grained compact 

 limestone ; the Conularia is also an almost constant associate of these shells, and rarely 

 found in other situations. These sliells, 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. 



