THE LOWER SILURIAN PERIOD. 113 



Lastly, the Lower Silurian Rocks have yielded a vast num- 

 ber of chambered shells, referable to animals which belong 

 to the same great division as the Cuttle-fishes (the Cephalo- 

 poda), and of which the Pearly Nautilus is the only living 

 representative at the present day. In this group of Cephalopods 

 the animal possesses a well-developed external shell, which is 

 divided into chambers by shelly partitions ("septa"). The 

 animal lives in the last-formed and largest chamber of the 



Fig. 53. Different views of Bellerophon Argo, Trenton Limestone, Canada. 

 (After Billings.) 



shell, to which it is organically connected by muscular attach- 

 ments. The head is furnished with long muscular processes or 

 " arms, " and can be protruded from the mouth of the shell at 

 will, or again withdrawn within it. We learn, also, from the 

 Pearly Nautilus, that these animals must have possessed two 

 pairs of breathing organs or " gills ; " hence all these forms are 

 grouped together under the name of the " Tetrabranchiate " 



Fig. 54. Different views of Machirea crenulata, Quebec Group, Newfoundland. 

 (After Billings. ) 



Cephalopods (Gr. tetra, four; branghia, gills). On the other hand, 

 the ordinary Cuttle-fishes and Calamaries either possess an 

 8 



