14 PALEONTOLOGY OF NEW JERSEY. 
Besides the absence of these parts, which often present important generic 
features in the embryonic nucleus, there is an almost total absence of the 
anterior beak and front of all shells, which so generally possess important 
generic features. Occasionally the casts retain to some extent the surface 
markings of the species. This is particularly the case in the Eocene marls, 
but in the Cretaceous beds it is a rare feature; consequently it has been 
necessary to resort to various methods for determining the generic relations. 
For specific differences the student is compelled to rely chiefly upon the form 
of the volutions and upon the proportions of parts, such as the compact- 
ness of the coil; the differences in the apical angle; the size and strength 
of the axis of the shell or columella, which is ascertained from the size and 
form of the cavity left by its removal; the imprints of folds left upon that 
part of the matrix which has rested against the columella, and various other 
features according to the conditions of the specimens. In some cases where 
much doubt of generic relations has been felt, resort has been had to arti- 
ficial casts made by filling the interior of recent shells with plaster and 
dissolving away the shell by means of acid to procure the cast of the 
interior. I have thus used every means within my reach to render the 
generic determinations of these casts as reliable as possible. Concerning 
many of these I am somewhat in doubt, but as a general thing they may 
be relied upon with a considerable degree of certainty. 
In regard to the authenticity of localities assigned to the several 
species, the remarks made in the previous volume’ hold good here, as 
the same collections which furnished material for that volume have been 
drawn on as largely for the material in this. I have used the types of all 
previously described species wherever I could find them, even to fragments 
preserving the least particle of character. The species of Gasteropoda are 
much better represented by individuals in the collections used than were the 
Lamellibranchiates, although of many of them there is but a single example 
known in the collections to which I have had access. The illustrations of 
many of the species are necessarily quite restricted and meager in quantity. 
Many species have been recognized, but not used, simply because of the 
1Tbid., p. xviii, 
