Fossils of the Lower Silurian RocJcs of Canada, 45 



more extreme forms miglit be regarded as distinct, did we not find 

 numerous intermediate ones, showing a gradation from one to the other. 

 The shells, more or less convex, depending on pressure, which sometimes 

 obliterates the prominent oblique elevation extending backwards from the 

 beak. Owing to the same cause, also, the beak is more or less prominent ; 

 and the pressure in different directions changes the form of the shell.'' 



This fossil is everywhere found in the central and Jiigher part of the 

 Hudson River group. It occurs at Toronto. In England it is not 

 uncommon in the Caradoc sandstone. 



Fig. 9. Fig. 10. 



Fig. 9 — Isotelus gigas. 

 " 10 — Calymene senaria. 



The two trilobites above figured, appear to have swarmed in prodigious 

 multitudes in the seas of the Trenton limestone period. Judging from the 

 abundance of their remains in every part of the great bed of sediment, 

 which constitutes the formation, the ocean was continually filled with shoals 

 of these creatures, similar to the thickend droves of herring and mackerel 

 which are to be met with in the Atlantic at the present day. There were 

 no true fish, or such as have an internal bony skeleton, but in company with 

 the trilobites great numbers of orthoceratites — marine animals, with their 

 bodies inclosed in long tube-like chambered shells, and their heads furnished 

 with powerful arms for capturing their prey, ruled with unlimited sway over 

 all the less formidable tribes of that ancient deep. These two tribes, then 

 the reigning powers among the living things of this world, were in full 

 bloom of strength during the Silurian epoch, but shortly after began to 

 decline, and finally disappeared for ever, about the time of the commence- 

 ment of the carboniferous period. The most abundant form in the earlier 

 Silurian ocean of America, was the Isotelus gigas, a figure of which, upon 

 u reduced scale, is given above. All that remains to us of this extraor- 

 dinary animal is the crustaceous jointed armour with which its head, back 

 and tail were covered. The same remark applies to all the trilobites. It is 

 only the shelly upper covering that has been preserved, while no traces of 

 parts which might show the form of the abdomen, feet, or other organs upon 

 the under side, have ever been discovered, and, consequently, we are as yet 

 totally without any, save conjectural ideas upon the principal portion 

 of their structure. 



