112 E VOL UTION OF TO-DA Y. 



can find no instance where a true fossil connecting- 

 link appears later than the groups connected by it. 

 In short, the geological location of fossils is exactly 

 that which the theory of descent would lead us to 

 expect. 



For reasons already considered, the number of 

 synthetic forms found among invertebrate fossils is 

 not so great as that of the vertebrates. But still 

 they are not absent, though they are few and scat- 

 tering. It is not possible in any case among inver- 

 tebrates to build a connected history such as we 

 have seen can be done with some vertebrate fami- 

 lies. The scattered connecting forms only enable 

 us to get a hint here and there as to the probable 

 development. The Crustacea form an interesting 

 group. The existing crabs can be traced back 

 through various simple form to the Oolitic rocks, 

 where they seem to merge into the less highly devel- 

 oped group Anomoura, a group in many respects in- 

 termediate between them and the Macroura (lobsters 

 and shrimps). Still further back the Anomoura dis- 

 appear, and the group of Macroura take their place 

 in the Carboniferous. And still earlier these are 

 united to the Phyllopods by Silurian forms (Ceratro- 

 caris). In another line the ancient Euryperids are 

 united to the family to which the king-crab belongs, 

 by a fossil called Hemiaspis, while the king-crabs are 

 united to the ancient Trilobdes by another fossil 

 (PrestwichSa). Alexander Agassiz has very carefully 

 studied the whole series of fossil forms of sea- 

 urchins. From the fact that the classes of this group 

 were already differentiated in the Silurian age, it 



