TRENTON LIMESTONE. 
93 
BRACHIOPODA OF THE TRENTON LIMESTONE. 
Plates XXX., XXXI., XXXII. & XXXIII. 
The Bkachiopoda of this period are among the most numerous and prominent fossils of 
the rock. Several species are so abundant as to constitute of themselves entire strata, and 
others cover extensive surfaces of the thin layers. Those which occur in the greatest 
profusion in New-York, are the Orthis testudinaria (identical with the species of the same 
name in Europe), and Leptcena sericea ; while other forms of Lepta:na are scarcely less 
abundant in some localities. In the western extension of the same rock, the Delthyris lynx 
and Atrypa protea are equally abundant; but these species are comparatively rare in New- 
York, and are never prominent species in any localities east of Ohio. 
The species of this order are comparatively constant and reliable throughout extensive 
areas, and the typical species of this rock in New-York are equally characteristic of strata of 
the same age in Ohio and Indiana. It is true, that both in this State and in all the western 
localities, many of these species are equally characteristic of the upper part of this great 
group, viz. the Hudson-river rocks. In New-York, some of the same species which mark 
the Trenton limestone are equally abundant in the shaly sandstones of the Hudson-river 
group ; while in the latter, we do not know of more than two species, at the present time, 
distinct from those in the limestone below, and these may yet be found in a lower position. 
It is from the occurrence of these species of Brachiopoda in the Hudson-river group, 
together with species of other genera, that I am induced to unite the whole, as before : 
stated, in one great series, though the lithological characters are so different in the two 
extremities. Among others, the Lingula: hold a prominent place ; this limestone containing 
half as many as we know, at present, in all the other palaeozoic rocks of America. This is 
the more interesting, since this genus, containing species of the same type, exists in the 
waters of our present seas, together with the Orbicula, another prominent fossil of this 
period ; while the other genera are unknown as existing forms. 
Although the number of species of this order isuiearly as great as of any other group of 
equal thickness, still it is probable, from what we already know, that the number will be 
increased at least one half, and perhaps doubled, when the Brachiopoda of the same period 
in the West shall be fully known. It is probable that we shall yet discover species in New- 
York which may swell the present number considerably, though we can scarcely anticipate 
that we shall ever discover all those species which flourished in the more favorable conditions 
of the western ocean of that period. 
