TRENTON LIMESTONE. 149 
ACEPHALA OF THE TRENTON LIMESTONE. 
Pirates XXXIV., XXXV. & XXXVI. 
In the fossils of this class, we find a much greater difficulty in the determination of 
genera and species, than in the Bracuiopopa, where the external characters alone are often 
sufficient for this purpose. The Acrpuata, also, are usually less perfectly preserved, and 
separate from the rock with greater difficulty, rarely presenting us with any of those 
characters on which generic distinctions in such shells are commonly founded. In far the 
greater number of instances they occur as separated valves imbedded in the stone, and in 
parts of the rock of such a nature that the shell is partially or entirely exfoliated, obliterating 
even the means of identifying them by this slender aid. The form of the fossil, therefore, 
is generally the principal or only means of its identification; and every paleontologist 
knows how liable this is to variation, from pressure, and other causes of which we can 
have but a slight knowledge. It will not be surprising, therefore, that differences of opinion 
may exist as to the propriety of the reference of many of these species to recent genera, or 
to genera constituted to receive analogous fossil species of a later epoch, which have been 
studied with better advantages. 
Notwithstanding the general paucity of species of this order, I am able to present a much 
larger number than have ever before been known from the Lower Silurian period, where 
they have been regarded as very rare. Among these there are two or three forms which 
present characters requiring a generic designation, and which appear to me not only thus 
distinct, but equally distinct from analogous forms in the later deposits. Acting therefore 
upon the conviction that more good can be accomplished by attempting to discriminate, 
than to risk confounding with what is really unlike, I have attempted to point out characters 
which may serve at least to distinguish several fossils now generally referred to recent 
genera, and from which all our knowledge, as well as analogy, would teach us to separate 
them. 
The Genus Cypricarpia has heretofore been the receptacle of a large number of species 
widely differing from each other, and few, if any, of which probably belong to the genus. 
The name of Nucuta has also been appended to shells, having few or none of the essential 
characters of that genus. Avicuxa, in like manner, has been the name applied to many 
shells possessing a remote analogy to some forms of that genus; and Inoceramus, a genus 
of the Oolitic and Cretaceous periods, has received both Silurian and Carboniferous species. 
It is true, as just observed, that in many instances it is almost impossible to ascertain 
with certainty the essential generic characters of these shells ; but in such cases we should 
study with more care the external form, and the structure of the shell, which may enable 
us to arrange the fossil species in natural groups. 
