252 MANUAL OF ZOOLOGY. 



3. The higher Crustacea are either entirely wanting or are 

 sparingly represented in the colder regions of the globe, but 

 increase rapidly in relative numbers as the equator is ap- 

 proached. 



4. The size attained by the Crustacea is greater on the aver- 

 age in warm regions than in colder climates. 



5. The special points of structure which are characteristic 

 of the different groups of Crustacea are more strongly mani- 

 fested in the warmer regions of the globe. 



6. There exists a decided relation between the temperature 

 of any given region and the character of its Crustacean fauna ; 

 similar generic forms being usually found occupying regions of 

 the same climatal character. 



DISTRIBUTION OF CRUSTACEA IN TIME. As regards the 

 general distribution of the Crustacea in time, remains of the 

 class are comparatively abundant in all formations except the 

 very oldest ; as might have been expected from the generally 

 chitinous or sub-calcareous nature of their integuments and 

 their aquatic habits. Owing also to their habit of periodically 

 casting their shell, a single individual may leave repeated 

 traces of himself, and the number of fossils may considerably 

 exceed that of the individuals which actually underwent fossili- 

 sation. The Crustaceans appear to have commenced their 

 existence in the Cambrian period, remains of numbers of this 

 class being tolerably abundant in the higher portion of this 

 formation. The Palaeozoic formations, taken as a. whole, are 

 characterised by the predominance of the orders Trilobita, 

 Eurypterida, Ostracoda, and Phyllopoda, of which the two former 

 are exclusively confined to this period. All the other orders of 

 Crustacea, which have left any traces of their past existence 

 at all, appear to have come into existence before the close of 

 the Palaeozoic period. Upon the whole, however, there has 

 been a marked progression in proceeding from the older 

 formations to the present day. The Trilobites and Eurypterids 

 of the older Palaeozoic Rocks, though highly organised so far 

 as their type is concerned, are in many respects inferior to later 

 forms, whilst they present some striking points of resemblance 

 to the larval forms of the higher groups. The great group of 

 the Stalk-eyed Crustaceans undoubtedly the highest of the 

 entire class is not represented at all till we reach the Car- 

 boniferous Rocks ; and it is not till we come into the Second- 

 ary period that we find any great development of this group, 

 whilst its abundance increases to a marked extent in the 

 Tertiary period, and it attains its maximum at the present day. 

 Similarly, of the two orders of the Merostomata, the Eurypterida 



