514 SINGIQVIRS, SHOW, SI GHONEIN IGS 
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, modern biology can give us 
little help. Weare thus toa greater extent than with the Brach- 
iopoda thrown entirely on the ontogeny of fossils, and in this case, 
too, the various stages must be worked out from separate indi- 
viduals. Many naturalists, beginning with Barrande, have worked 
on the ontogeny of trilobites, have described various stages, 
sometimes as larvee, 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 
overcome these difficulties, presenting his results in a recent 
yy 
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 various 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. ? 
Mollusca.—Of the Mollusca only the Pelecypoda and the Ceph- 
alopoda are of use to the student of paleontogeny, for the Gas- 
tropoda have not been classified in a satisfactory manner, and the 
larval stages even of living forms not well studied. 
Pelecypoda.—Almost all that has been done in comparing 
genera of Pelecypoda with stages of growth is the work of Dr. R. 
T. Jackson,3 who has shown that they all go through a phylem- 
bryonic stage, prodissoconch, analogous to the protegulum of 
tAmer. Geol., Vol. XVI, Sept. 1895. 
2 Amer. Jour. Sci., Feb. and March 1897, ‘‘Outline of a Natural Classification of 
Trilobites.” 
3Mem. Boston Soc. Nat. History, Vol. IV, No. 8, 1890, “Phylogeny of the Pele- 
cypoda.” 
