PRE-CAMBRIAN FAUNAS 121 



been suited for preservation. Although most phyla, 

 and a considerable proportion of orders, of Invertebrates 

 are represented in the Cambrian, their numbers are 

 few when compared with those known from the 

 Ordovician. Fossils are locally common, even in the 

 lower strata of the Cambrian system, but abundance is 

 the attribute of an insignificant number of species. 

 Extraordinary discoveries, such as those made in the 

 Middle Cambrian of British Columbia, show that there 

 was no lack of soft-bodied animals at that time ; but 

 such organisms are incapable of preservation under 

 normal circumstances. Analysis of the fossils of the 

 Cambrian suggests that chitin, rather than calcareous 

 or siliceous matter, was the prevalent " reinforcing " 

 substance ; and chitin is usually too delicate in texture 

 for satisfactory fossilization. The three groups of 

 Invertebrates best represented in the Cambrian are the 

 Brachiopods (chiefly Atremata and Neotremata), the 

 Pteropods and the Trilobites. The lower orders of 

 Brachiopods secrete shells which are largely corneous 

 (e.g. Linguld), and the types prevalent in Cambrian 

 times seem, on the whole, to have possessed this quality, 

 with the additional character of smallness. The majority 

 of the Pteropods ("sea-moths") are provided with 

 excessively delicate shells, in which mineral matter is 

 often entirely wanting. The Cambrian members of 

 the group (e.g. Hyolithes), though possessing partly 

 calcareous shells, seem to have been thinly clad. The 

 Trilobites, like other Arthropods, had chitinous cara- 

 paces, in which only a small percentage of mineral 

 matter was present. 



Even the relatively scarce Cambrian representatives 

 of other phyla usually show imperfect or flimsy 

 mineralized structures, in striking contrast with the 

 often massive shells and skeletons of their Ordovician 



