260 THE LIFE OF CRUSTACEA 



genera and species of Trilobites present great differ- 

 ences in the form and ornamentation of the dorsal 

 surface of the body, and it is probable that con- 

 siderable differences may also have existed in the 

 structure of the limbs, which are only known in two 

 or three species. Some Trilobites are among the 

 most ancient of known fossils, being found in rocks 

 of the Lower Cambrian epoch. The group reaches 

 its maximum development in the Ordovician, and 

 the number of the species and size of the individuals 

 gradually diminish through the Silurian and Devonian 

 till they become extinct at the close of the Carbon- 

 iferous epoch, except for a single species found in 

 rocks of Permian age in America. 



Although zoologists are not all agreed as to the 

 precise systematic place to be assigned to the Trilo- 

 bites, there can be little doubt that they were related 

 more or less closely to the most primitive Crustacea, 

 and they are of special interest as preserving for us 

 the stage in which the second pair of appendages 

 were still used as biting jaws, and had not moved 

 forwards in front of the mouth to become antennae, 

 as in all living Crustacea. 



Contemporary with some of the earliest Trilobites, 

 however, are undoubted Crustacea, which, so far as 

 we know their structure, are not very different from 

 types now living. In the Cambrian epoch the 

 Branchiopoda appear to be represented by Protocaris, 

 which in its general form resembles A pus ; and 



