12 BULLETIX OF THE NATURAL HISTORY SOCIETY. 



Cambrian Faunas of the Interior Sea and op the Atlantic 



Basin. 



Meanwhile there had been growing up in the interior of 

 America a fauna of an entirely different type. There the 

 warm seas had nourished crustaceans of genera differing from 

 those of the Atlantic coast ; and the molluscs had shown 

 themselves capable of variation in several directions. On the 

 one hand Maclureas presented themselves as one of the most 

 important early developments of the sea snails, and various 

 forms of Orthocerata showed the early phases of the class 

 from which the Nautili subsequently sprang. These forms 

 were borne on the warm waters which laved the eastern 

 shores of the North American continental nucleus, from New 

 York to the Strait of Belle Isle, and its western shore from 

 Manitoba to Boothia Felix and Grinnell Land, thus reaching 

 Europe by the circuitous route of the Arctic circle. Shallow 

 waters and warm currents enabled these shells born in the 

 Mediterranean of America to penetrate to the pole itself, and 

 this before they had been able to invade the chilled waters of 

 the Atlantic coast. However, we should remember that the 

 continental lands were then probably of less extent than now, 

 and the earth's atmosphere denser, conditions which would 

 favor a more uniform distribution of heat over the earth's 

 surface and enable the animals of the tropical seas to invade 

 the Arctic circle. 



While these early sea snails and straight nautiloid forms 

 had been growing up and developing in the warm seas of the 

 interior, another type of beings had been fitting themselves 

 to the very different conditions of existence in the North 

 Atlantic. These, the Graptolites, were roving creatures, 

 fitted to propagate their kind in the open sea. Through the 

 subsequent ages we find them gradually simplifying their 

 structure, so that the colonies of many branches of the earliest 

 times are reduced at last to colonies of one stem without a 

 branch, and with rows of cells on one side of the stem only. 

 In modern times we have the Sertularians, branching colonies 

 of minute creatures as representatives of the Graptolites. In 



