HUDSON-RIVER GROUP. 285 



BRACHIOPODA OF THE UTICA SLATE AND HUDSON-RIVER GROUP. 



Plate LXXIX. 



Few species of this class are known in tlie Utica slate, and the individuals rarely found 

 are specifically identical with those of the Trenton limestone. As the shales become less 

 carbonaceous and lighter colored, with an admixture of arenaceous and calcareous matter, 

 some of the species so abundant in the Trenton limestone again make their appearance 

 in great force, and characterize the strata in almost all localities. Although, from the 

 nature of the mass, they are in a different condition, they nevertheless preserve all the 

 peculiarities of the same species in a lower position ; while the absence of the shell, and 

 the abundance of moulds of the interior and exterior surface, have induced an opinion 

 that (here are several distinct species. This remark is peculiarly applicable to the Leptama 

 alternata, which is rarely found preserving its shell entire, while the impressions of the 

 outer and inner surface are abundant. This species has already (pages 102, 103 and 104, 

 PI. 31 and 31 a of this volume) been fully illustrated, and a few figures presenting its 

 appearance and character in this group are given in the present connection. 



It should be borne in mind, however, that comparatively few of the Trenton limestone 

 species of Braciiiopoda are found in the strata of this group, the larger portion ceasing 

 their existence with that rock. The forms given on Plate Ixxix are nearly all that usually 

 occur; the Lcptcena alternata, L. scricea and Orthis testudinaria being quite abundant, 

 while the Atrypa and Lingula are rare. At the same time we find two other species of 

 Orthis, wliich have not been observed in the limestone below. The same horizon, in the 

 western extension of the formation, gives us not only the Trenton limestone species, 

 but also several others not known within New-York. The few species occurring in this 

 position in New- York, differing from those known in the Trenton limestone, are not therefore 

 to be regarded as offering any important distinction between the two portions of the group ; 

 for we shall doubtless yet find many more in the lower limestones than we now know. 



133. 7. LINGULA QUADRAT A. 



Pl. LXXIX. Figs. 1 a,b. 



Reference pag. 90, pi. 30, fig. 4, of this volume. 



Lingula rectiliileris, Conrac, MS. Emmons, Geol. Report, 1312, pag. 39^, fig. 0. 



I am unable to perceive any essential difference between this shell and the /.. quadrata 

 of the Trenton limestone. The figure given by Prof. Emmons has the sides straighter and 

 the upper extremity more pointed than the original specimen. In two specimens examined 

 there is a slight difference in the form, owing in part to compression ; but there is no more 



