TRILOBITES AND OTHER FOSSIL CRUSTACEA. 203 



In these animals, curious and wonderful as they are in 



their power of adaptation, we find a class which are 



zoologically more highly organ- 



ized in their larval state than in 



their adult condition. The doc- 



trine of evolution admits of 'retro- 



gression as well as of progression, 



and the Cirripedia are all, with- 



out an exception, illustrations of 



the former. Consequently, for 



the philosophical student to find 



fossil specimens of a group illus- 



trating retrogression is an inter- 



esting fact. 



It would appear as if the 

 stalked barnacles (Lepadidcs) pre- 

 ceded in geological time the 

 sessile kinds. Thus we find a 



_ , .. r i i Fig. 162. Scalpellnni vulgare 



genus of the former so far back (recent). 



as the Upper Silurian rocks, in 



which Turrilepas occurs. In the 



Rhsetic strata we have Polli- 



cipes ; we come across them fre- 



quently in true marine strata 



(and all of this group are of 



a thoroughly marine character), Fig- 163. Bai 



. 1,1 r\ TL A. j.1. Figs. 162, 163. Specimens of 



through the Oolites, mtO the recent stalked and sessile 



White Chalk, where Darwin 



mentioned thirty-two species as having been dis- 



