EVOLUTION OF FOSSIL CEPHALOPODA. 239 
ogy, 1896, may be taken as strictly biogenetic so far as 
the data now at hand make such a thing possible. 
Crustacea.—The only Crustacea that are useful for the 
study of paleontogeny are the trilobites, and since they 
are all extinct without leaving any descendants, mod- 
ern biology can give us little help. We are thus to 
a greater extent than with the Brachiopoda thrown en- 
tirely on the ontogeny of fossils, and in this case, too, 
the various stages must be worked out from separate in- 
dividuals. Many naturalists, beginning with Barrande, 
have worked on the ontogeny of trilobites, have de- 
scribed various stages, sometimes as larve, sometimes 
as adult genera or species, but they met with seemingly 
insuperable difficulties in correlating these stages with 
the genealogy. Dr. C. E. Beecher, however, has over- 
come these difficulties, presenting his results in a recent 
paper on The Larval Stages of Trilobites,* in which 
he shows that all trilobites go through a phylembryonic 
stage, protaspis, homologous to the protonauplius of the 
higher Crustacea. While no known genera are exactly 
like the protaspis, still there are several that retain 
many of its features, After the protaspis stage the vari- 
ous groups of genera develop in different directions, 
but all go through larval stages analogous to generic 
changes in their group. The protaspis itself of the 
later groups becomes more complicated by acceleration 
of development, but always retains its essential features. 
By means of this study Dr. Beecher has been able to 
give the beginning of a one genetic classification of 
trilobites.+ 
Mollusca.—Of the aribhesca only the Pelecypoda and 
the Cephalopoda are of use to the student of paleontog- 
* Amer. Geol., vol. xvi, September, 1895. 
+ Amer. Jour. Sci., February and March, 1897. Outline of a 
Natural Classification of Trilobites.” 
17 
