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 o{ retro- 

 gression as well as o( 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 {Lepadidce) pre- 

 ceded in geological time the 

 sessile kinds. Thus we find a 

 genus of the former so far back '^'^ ^*~(rec/nV'"^"^'"^ 

 as the Upper Silurian rocks, in 

 which Turrilepas occurs. In the 

 Rhaetic 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. x(>i.—Baiamis porcatus. 

 through the Oolites, into the ^^^^c£%^^^^^^ 

 White Chalk, where Darwin 

 mentioned thirty-two species as having been dis- 



