" HUDSON-RIVER GROUP. 285 
BRACHIOPODA OF THE UTICA SLATE AND HUDSON-RIVER GROUP. 
Pirate LXXIX. 
Few species of this class are known in the 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 there are several distinct species. This remark is peculiarly applicable to the Leptena 
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, 
Pl. 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 Bracutopopa 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 Leptena alternata, L. sericea and Orthis testudinaria being quite abundant, 
while the Atrypa and Lingula are rare. At the same time we find two other species of 
Orthis, which 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 QUADRATA. 
Pu. LXXIX. Figs. 1 a, b. 
Reference pag. 96, pl. 30, fig. 4, of this volume. 
Lingula rectilateris, Conrap, MS. Emmons, Geol. Report, 1842, pag. 399, fig. 6. 
I am unable to perceive any essential difference between this shell and the L. 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 
