SUMMARY. 



change in feeding habits, appendages were developed, and, due perhaps to physiological 

 change induced by changed food, a shell was secreted on the dorsal surface, covering the 

 whole body. Such a shell need not have been segmented, and, in fact, the stiffer the shell, 

 the more reason for development of the appendages. Activity as a swimming and crawling 

 animal tended to break up the dorsal test into segments corresponding to those of the soft 

 parts, and, by adaptation, a floating animal became a crawling one, with consequent change 



Fig. 36. Naraoia 

 comfacta Walcott. 

 An outline of the 

 test, after Walcott. 

 Natural size. 



Fig. 37Pagetia 

 cfytio Walcott. An 

 eodiscid with com- 

 pound eyes. After 

 Walcott. X 5. 



Fig. 38. Asaphis- 

 cus wheeleri Meek. 

 A representative 

 trilobite of the 

 Middle Cambrian 

 of the Pacific 

 province. After 

 Meek. X Vi. 



Fig. 39. Padeumias 

 robsonensis Burling. Re- 

 stored from a photograph 

 published by Burling. 

 X'A. 



Fig. 40. Robergia sp. 

 Restored from fragments 

 found in the Athens shale 

 (Lower Middle Ordovi- 

 cian), at Saltville, Va. 

 Natural size. 



from a form like that of Naraoia to one like Pccdeumias. (See figs. 36-40.) A contin- 

 uation of this line of development by breaking up and loss of the dorsal test led through 

 forms similar to Marrella to the Branchiopoda of the Cambrian, in which not only is there 

 great reduction in the test, but also loss of appendages. The origin of the carapace is still 

 obscure, but Bernard (1892, p. 214, fig. 48) has already pointed out that some trilobites, 

 Acidaspid;e particularly, have backward projecting spines on the posterior margin of the 

 cephalon, which suggest the possibility of the production of such a shield, and in Marrella 

 such spines are so extravagantly developed as almost to confirm the probability of such 



