APPEARANCE OF VERTEBRATE ANIMALS. 121 



and that the ocean was still dominated by Invertebrates long 

 after the fishes had become supreme in bays, estuaries, and 

 rivers. 



The first fishes that we certainly know are the Ganoids and 

 Sliarks, which appear near the close of the Upper Silurian, in 

 the English Ludlow for example (Fig 104). The Ganoids found 

 here all belong to an extinct group, characterised by the covering 

 of the head and anterior part of the body with large bony 

 plates. They are mostl> small fishes, and probably fed at the 

 bottom, and used their long or rounded bony snouts for 



l''\c,. 1014. — a, Head-shield of an Upper Silurian fis,h {Cyat/ias^/s). b. Spine of a Silurian 

 Shark (Onchtis icntti-striatus, Agass.). c, Scales of Thclodns. 



grubbing in the mud for food. In this respect they present 

 a singular resemblance to the Trilobites, so that wc seem to 

 have here animals of an entirely new type, the Vertebrate, and 

 with bony instead of shelly coverings, taking up the role and, 

 to some extent, the external form of a group about to pass away. 

 Yet I presume that no derivationist would be hardy enough to 

 affirm that the Trilobites could have been the ancestors of 

 these fishes. Nor indeed is any ancestry even hypothetically 

 known for them, for even the doubtful Lampreys of the Lower 

 Silurian are too remote in structure to be used in that way. 



