THE LOWER AND UPPER SILURIAN AGES. 75 



their shagreen -like skin, strong bony spines, and 

 trenchant teeth, to have much resembled our modern 

 sharks, or rather the dog-fishes, for they were of 

 small size. One genus (Pteraspis), apparently the 

 oldest of the whole, belongs, however, to a tribe of 

 mailed fishes allied to some of those of the old red 

 sandstone. In both cases the groups of fishes repre- 

 senting the first known appearance of the vertebrates 

 were allied to tribes of somewhat high organization 

 in that class ; and they asserted their claims to domi- 

 nancy by being predaceous and carnivorous creatures, 

 which must have rendered themselves formidable 

 to their invertebrate contemporaries. Coprolites, or 

 fossil masses of excrement, which are found with 

 them, indicate that they chased and devoured ortho- 

 ceratites and sea-snails of various kinds, and snapped 

 Lingula3 and crinoids from their stalks ; and we can 

 well imagine that these creatures, when once intro- 

 duced, found themselves in rich pasture and increased 

 accordingly. Space prevents us from following further 

 our pictures of the animal life of the great Silurian 

 era, the monuments of which were first discovered 

 by two of England's greatest geologists, Murchison 

 and Sedgwick. How imperfect such a notice must 

 be, may be learned from the fact that Dr. Bigsby, in 

 his "Thesaurus Siluricus/' in 1868, catalogues 8,897 

 Silurian species, while only 972 are known in the 

 Primordial. 



Our illustration, carefully studied, may do more to 

 present to the reader the teeming swarms of the 



