Charles Emerson Beecher. 419 



the phylo-typembryo nor the phylo-phylembryo, but is really 

 the adult equivalent to an early segmented stage of the higher 

 genera." Beecher divided the early stages of development in 

 trilobites as follows : " .Nauplius (Cephalon predominating, 

 other parts not separated from it), Phylembryonic (Cephalon 

 distinct, thorax nothing, pygidium distinct), Nepionic with as 

 many stages as there are normal thoracic segments (Cephalon 

 distinct, thorax incomplete, pygidium distinct), Keanic (Cepha- 

 lon, thorax and pygidium all distinct and complete; growth 

 incomplete), Ephebic (all parts complete and full size attained)." 



The protaspis is homologous to the crustacean nauplius, 

 which had "potentially five cephalic segments bearing append- 

 ages, which should therefore be taken as characteristic of a 

 protonauplius. The nauplius is a modified crustacean larva. 

 The protaspis more nearly represents the primitive ancestral 

 larval form for the class, and approximates the protonauplius." 



The basis for Beecher's classification of the trilobites is the 

 application, for the first time, of the law of morphogenesis, or 

 the recapitulation theory. He observed that in the first or 

 unsegmented stage of the most primitive trilobites there are 

 neither dorsal free cheeks nor eyes, but that in some of the 

 later forms both the eyes and free cheeks have migrated to the 

 anterior margin or may even have progressed a little posteriorly 

 down the dorsal side of the protaspis. This led him to under- 

 take a study of all trilobite genera, more than two hundred in 

 number, and it was seen that these could be arranged in three 

 groups on the basis of the nature and position of the free 

 cheeks. In the most primitive order, or the Hypoparia, there 

 are " free cheeks forming a continuous marginal ventral plate 

 of the cephalon, and in some forms also extending over the 

 dorsal side at the genal angles." In the Opisthoparia the 

 dorsal ''free cheeks include the genal angles, thus cutting off 

 more or less of the pleura of the occipital segment ; " while in 

 the Proparia, or the last order to arise, "the pleura of the 

 occipital segment extend the full width of the base of the 

 cephalon, embracing the genal angles." 



There is much diversity of opinion regarding the rank of 

 trilobites in a classification of the Crustacea. Beecher regarded 

 them as a sub-class and as equal in rank to the Entomostraca 

 and Malacostraca. "In nearly every particular the trilobite is 

 very primitive, and closely agrees with the theoretical crusta- 



