THE CAMBRIAN PERIOD. 87 



lived. The vital organs seem to have occupied the central lobe 

 of the skeleton, by which they were protected ; and a series of 

 delicate leaf-like paddles, which probably served as respiratory 

 organs, would appear to have been carried on the under surface 

 of the thorax. That they had their enemies may be regarded 

 as certain ; \>ut we have no evidence that they were furnished 

 with any offensive weapons, or, indeed, with any means of 

 defence beyond their hard crust, and the power, possessed by 

 so many of them, of rolling themselves into a ball. An addi- 

 tional proof of the fact that they for the most part crawled 

 along the sea-bottom is found in the occurrence of tracks and 

 markings of various kinds, which can hardly be ascribed to 

 any other creatures with any show of probability. That this 

 is the true nature of some of the markings in question cannot 

 be doubted at all ; and in other cases no explanation so pro- 

 bable has yet been suggested. If, however, the tracks which 

 have been described from the Potsdam Sandstone of North 

 America under the name of Protichnites are really due to the 

 peregrinations of some Trilobite, they must have been pro- 

 duced by one of the largest examples of the order. 



As already said, the Cambrian Rocks are very rich in the 

 remains of Trilobites. In the lowest beds of the series (Long- 

 mynd Rocks), representatives of some half-dozen genera have 

 now been detected, including the dwarf Agnosttts and the giant 

 Paradoxides. In the higher beds, the number both of genera 

 and species is largely increased ; and from the great compara- 

 tive abundance of individuals, the Trilobites have every right 

 to be considered as the most characteristic fossils of the Cam- 

 brian period, the more so as the Cambrian species belong to 

 peculiar types, which, for the most part, died out before the 

 commencement of the Silurian epoch. 



All the remaining Cambrian fossils which demand any notice 

 here are members of one or other division of the great class 

 of the Mollusca, or " Shell-fish " properly so called. In the 

 Lower Cambrian Rocks the Lamp-shells (Brachiopoda) are the 

 principal or sole representatives of the class, and appear chiefly 

 in three interesting and important types namely, Lingiddla, 

 Distinct, and Obolella. Of these the last (fig. 32, /') is highly 

 characteristic of these ancient deposits ; whilst Discina is one 

 of those remarkable persistent types which, commencing at 

 this early period, has continued to be represented by varying 

 forms through all the intervening geological formations up to 

 the present day. Lingulclla (fig. 32, c], again, is closely allied 

 to the existing " Goose-bill " Lamp-shell (Lingula anatina), and 

 thus presents us with another example of an extremely long- 



