212 TOWARDS THE HUMAN FORM 



species and genera, permitting the delimitation of zoological 

 provinces even in the Cambrian Period. At that time the 

 species and even the genera of the Northern sea inlets were 

 distinct from those of the Tethys ; and the Circum-pacific 

 Ocean also had its own species. Sao, Conicephalus heberti and 

 C. levyi, and Paradoxides mediter ramus were unknown along the 

 coasts of the North Atlantic Continent, where Olenus was 

 common. During the Acadian Period the first Asaphidae 

 appeared in the Pacific. Thence they spread towards the 

 waters of the future Europe, which they did not actually reach 

 until the Ordovician Epoch. Dicellocephalus was characteristic 

 of the region of the Circum-pacific Ocean, which extended from 

 the western shores of America to Australia, but Olenus was 

 absent. During the Silurian Period certain Trilobites acquired 

 the faculty of rolling themselves up into a ball, as the Wood- 

 louse and Glomeris do to-day. Their genera and species 

 multiplied, but remained characteristic of distinct zones. 

 The existence of such zones, however, does not imply a 

 difference of climate, for the calcareous deposits did not change 

 in character. The floating Graptolites remained cosmopolitan, 

 and coral banks continued to form in the northern as in the 

 equatorial regions ; therefore, the temperature must have 

 remained high everywhere. Nevertheless after the Ordovician 

 Period the Trilobites enable us to distinguish clearly the 

 Northern European, American, and Bohemian regions. The two 

 first tend to become merged during the Gothlandian Period, 

 on account of exchanges taking place between these regions, 

 particularly between America and Europe, by way of the 

 Arctic Ocean. In the Devonian Epoch this communication 

 disappeared, and the American and European fauna once 

 more became distinct. The American fauna extended from 

 the United States to South America and the southern parts of 

 Africa, while the European extended over the rest of the world. 

 The Trilobites lose all importance in the Carboniferous 

 Period. These strange organisms approximated to Crustaceans 

 in the possession of antenna? ; but Crustaceans have two 

 pairs and their buccal appendages have definitely become 

 mandibles, jaws, and maxillae. As in the Trilobites, five pairs 

 of appendages, or six as in Limulus, if we count the peduncle 

 of the eyes in the higher Crustaceans, are employed either as 

 tactile organs, or for prehension and the trituration of food. 



