. PLAGIOSTOMI. 121 



be exemplified not known in other formations or in any 

 existing fishes. They may belong to Pterasjns, a placo-ganoid 

 fish, allied to Ccplicdaspis, the cephalic buckler of which 

 has been found in the transition beds overlying the Ludlow 

 rock. 



No detached teeth unequivocally referable to a plagios- 

 toinous genus, nor any true ganoid scale of a fish, have yet 

 been found in the formations that have revealed these earliest 

 known evidences of vertebrate animals. What then, it may be 

 asked, were the conditions under which so immense an extent, 

 as well as depth, of sediment was deposited — including 

 chambered Cephalopods, Gastropods, Lamellibranchs, Brachio- 

 pods, various and large trilobitic and entomostracous Crus- 

 taceans, with Crinoids, Polypes, and Protozoa — that precluded 

 the preservation of the fossilizable parts of fishes, if that 

 class of vertebrate animals had existed in numbers, and under 

 the variety of forms, comparable to those that people the 

 ocean at the present day 1 Bonitos now pursue flying-fishes 

 through the upper regions of an ocean as deep as any of 

 the Silurian seas of which the deposits afford an idea of 

 greatest depth. If fishes of cognate habits with the present 

 deep-sea fishes, under whatever difference of form such 

 Silurian fishes may have been manifested, had really 

 existed, we might reasonably expect to find the remains of 

 some of the countless generations that succeeded each 

 other during a period of time, sufficing for the gradual 

 deposition of sedimentary beds of thousands of feet in verti- 

 cal thickness. 



The evidences of plagiostomous fishes afforded by fossil 

 spines will be here pursued. In most of the existing cartila- 

 ginous fishes of this order the defensive spine which stands 

 erect in front of the dorsal fin is smooth ; such is the case in 

 the dog-fishes (Spinacidcc) in which each dorsal fin is fronted 

 with a spine. In the Port-Jackson sharks (CcstraciontiJcv) 



