SUMMARY. 
145 
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 stirrer 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 
compacta Walcott. 
An outline of the 
test, after Walcott. 
Natural size. ■ 
Fig. 37—Pagetia 
clytia 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 V2. 
Fig. 39. — Pccdcumias 
robsonensis Burling. Re- 
stored from a photograph 
published by Burling. 
X J 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 Pcrdeumias. (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, 
Acidaspkke 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 
