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 larvae, sometimes 

 as adult genera or species, but they met with seemingly 

 insuperable difificulties 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 truly genetic classification of 

 trilobites. f 



Mollusca. — Of the Mollusca only the Pelecypoda and 

 the Cephalopoda are of use to the student of paleontog- 



* Amer. Geol., vol. xvi, September, 1895. 



f Amer. Jour. Sci., February and March, 1897. Outline of a 

 Natural Classification of Trilobites." 

 17 



