314 Silurian Fossils, 



The remains of the most ancient tribes of the Cephalopoda are 

 exceedingly abundant in certain formations in Canada. The seas 

 of the Sikirian period, judging from the numbers of Orthoceratites 

 found in the rocks of that age, swarmed with these creatures ; 

 they all belonged to the order Tetrabranchiata. The Dibranchiata, 

 so powerful in the present oceans, did not come into existence 

 until many ages had elapsed after the Trenton limestone was 

 formed. The most common fossil Cephalopoda in Canada are the 

 Orthoceratites, an ideal figure of one of which is here given. 



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a 



c 



Fiof. 1. An Ortlioceratite* 



In order to understand the above figure, the reader will please 

 recollect what we have stated concerning the organization of the 

 modern Cuttlefish. Its body consists simply of a fleshy bag, in 

 some species of a globular shape in others cylindrical and having 

 the head and arms at one end. The object above figured is along 

 straight shell, open at one end at the letter a, and closed at h. 

 The space from & to c is divided into a number of compartments 

 by an equal number of transverse plates of shell, or septa as they 

 are called. These all communicate with each other by a tube 

 represented by the dotted lines along the centre. This tube is 

 called the siphuncle. From c to a is a space undivided and 

 constituting a single large chamber, which contained the body of 

 the animal. We have only to imagine the body of a modern 

 Cuttlefish placed in the large chamber from c to a ; and with its 

 head and legs or arms protruding from the mouth of the shell at 

 a, and we shall have formed a tolerably correct idea of a living 

 Orthoceratite. 



Certain species of those creatures in the ancient seas were of 

 a great size. In the collection of the Geological Survey at 

 Montreal, there are numerous specimens of the shells, which when 

 perfect must have been ten feet in length. Their arms may have 

 been of a corresponding length, and as the ocean swarmed with 

 them, bathing (had there been anybody in those remote ages to 

 bathe) would have been a dangerous recreation. 



* Copied from the "BuUclin de la Societe Geologique de France, Tome 12^ 

 Plaacbe V. 



