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 mp-de 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 



' Ibid., p. xviii. 



