168 PALEONTOLOGY OF NEW-YORK. 
or three specimens have been seen, and careful researches have been but poorly rewarded. 
It is evident from what we find in subsequent formations, that the nature of the sediment, 
or the condition of the ocean, was not favorable to their development during this period. 
In this respect they contrast with the species of Bracutopopa, which, for the most part, are 
abundant in one or more localities. 
An interesting comparison may be drawn between this and the subsequent period, or a 
later part of the same epoch, during which the shales of the Hudson-river group were 
deposited. In the latter group, the Bracutopopa known, with the exception of two species, 
are identical with those previously found in the Trenton limestone ; while the species of 
AcEPHALA are, for the most part, distinct from previously existing forms. Several species 
of those already described do appear in the shales and sandstones of the Hudson-river 
group; but a large majority of the forms are quite distinct, though belonging to the same 
genera. From these facts we are able to infer that the condition of the ocean, during the 
deposition of the calcareous strata, was more favorable to the development of the Bra- 
chiopoda, than of the Acephala ; also that the former are less susceptible to the influence 
of change, or more enduring than the latter, which, with the exception of three species, 
are not known in the succeeding strata. 
We shall find, also, in all subsequent paleozoic formations, that the calcareous strata 
have proved comparatively barren of the Acephala, though very prolific in the Brachiopoda; 
while, on the contrary, the periods of sandy and shaly deposits have been far more pro- 
ductive of the Acephala, with a comparatively larger proportion of Brachiopoda than we 
find of the Acephala in the calcareous deposits. We must take into account, however, in 
making this comparison, the character of both the calcareous and other deposits ; for, as 
in the case of the Trenton limestone, we may have a considerable proportion of shale ; and 
in formations where the other materials predominate, we shall find also a large amount of 
calcareous matter. In such instances we usually discover, as in the present case, that the 
Brachiopods are most abundant in the calcareous part (often, perhaps, constituting a large 
proportion of the same), while the Acephala occur in the shaly part of the formation. 
In the present instance, we have an opportunity of making a comparison with the same 
formation in its western extension, where the subdivisions made in New-York are scarcely 
recognizable, showing that the Acephala are less abundant throughout the formation than 
in New-York ; while several of the species known here in the higher part of the formation, 
occur there in a lower position, and others ascend perhaps quite to the upper limits of the 
formation, and there become more numerous than elsewhere. 
