CRUSTACEA OF THE PAST 261 



there are a variety of genera and species of Ostra- 

 coda, although, since their shells alone are preserved, 

 it is not possible to determine their exact relations 

 to existing forms. In the succeeding Ordovician 

 and Silurian epochs we first meet with the remains 

 of Barnacles, and it is interesting to note that some 

 of them are referred to the genera Pollicipes and 

 Scalpellum, which are represented by numerous 

 species in the seas of the present day. Along with 

 these, however, are some strange-looking forms 

 {TurrilepaSy etc.), having the body covered with rows 

 of overlapping plates. If these are really Cirripedes, 

 they must have differed considerably in structure 

 from the more modern types. 



The Malacostraca are more interesting from the 

 point of view of palaeontology than the other sub- 

 classes of Crustacea, since the evolution of the group 

 appears to have taken place within the period covered 

 by the fossil records, and it is possible to trace the 

 course of that evolution — at least, in its broad out' 

 lines. It has already been pointed out that the most 

 primitive of existing Malacostraca are the Phyllo- 

 carida (Nebalia and its allies), which are in several 

 respects intermediate between the higher Malacos- 

 traca and the Branchiopoda ; and it is interesting to 

 find that fossils apparently belonging to the Phyl- 

 locarida are found far earlier than any of the other 

 Malacostraca. In the Cambrian, and more abund- 

 antly in the Ordovician and Silurian, there are found 



