TRENTON LIMESTONE. 93 



BRACHIOPODA OF THE TRENTON LIMESTONE. 

 Plates XXX., XXXI., XXXII. & XXXIII. 



The Beachiopoda 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 Lepttena sericea ; while other forms of Lept^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 whicli 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 tlie two 

 extremities. Among others, the LmcuL^ hold a prominent place ; this limestone containing 

 half as many a» 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 is nearly 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. 



