LOWER PALAEOZOIC FAUNAS 149 



proportion of lime-salts is added. Although chitin is 

 a remarkably resistant substance, the usual delicacy of 

 Arthropod coverings and the loosely segmented quality 

 of many types, make their satisfactory preservation rare. 

 The normal mode of growth by ecdysis, while providing 

 a succession of envelopes from each individual, results 

 in production of much fragmentary material. A more 

 or less argillaceous tomb is necessary for prolonged 

 preservation ; but with such a matrix there is no limit 

 to the duration of chitin. 



From evidence available, there seems justification for 

 belief that Arthropods have been relatively as abundant 

 in all recorded periods of the past as they are to-day. 

 In the Palaeozoic era Trilobites seem to have more than 

 filled the position occupied by Crabs and Lobsters at 

 the present time; while small Ostracoda are scattered 

 through suitable matrices with equal profusion in the 

 Ordovician and Cretaceous. Insects are ill-adapted for 

 fossilization ; the considerable numbers known, even from 

 the Carboniferous, are suggestive of their existence in 

 vast swarms. It is a striking fact that the number and 

 variety of large Arthropods known from the Palaeozoic 

 are much in excess of those of later faunas. With few 

 exceptions Cambrian forms were Crustacea ; all Lower 

 Palaeozoic types were Branchiata. Myriopods and 

 Insects are unknown before the Devonian. 



Trilobites are among the most familiar fossils of the 

 Palaeozoic. In spite of their apparent complexity, these 

 extinct Crustaceans were among the simplest forms of 

 the phylum. The long succession of separate, similarly 

 specialized segments that characterized most Cambrian 

 types can be compared with the simple repetition of 

 parts in a caterpillar. In most Trilobite lineages there 

 appears a progressive tendency towards incorporation of 

 more segments into the pygidium (see PI. viii. fig. 4) 



