OF THE EARLIER VERTEBRATA. 57 



tae order in which patches and plates occurred. And then, 

 though not without some labour, I succeeded in tracing the 

 buckler with which they were associated to the Bi'pterus^ — 

 a fish which, though it has engaged the attention of both 

 Cuvier and Agassiz, has not yet been adequately restored. 

 It is on an ill-preserved Orkney specimen of the cranial 

 buckler of this ganoid that the ichthyologist has founded his 

 genus Polyphractus ; while groupes of its palatal teeth from 

 the Old Red of Russia he refers to a supposed placoid, — the 

 Ctenodus. But in the earlier ages of palseontological re- 

 search, mistakes of this character are wholly unavoidable. 

 The palaeontologist who did avoid them would be either very- 

 unobservant, or at once very rash and very fortunate in his 

 guesses. If, ere an entire skeleton of the Ichthyosaurus had 

 turned up, there had been found in different localities, in the 

 Liasic formation, a beak like that of a porpoise, teeth like 

 those of a crocodile, a head and sternum like those of a lizard, 

 paddles like those of a cetacean, and vertebrae like those of a 

 fish, it would have been greatly more judicious, and more in 

 accordance with the existing analogies, to have erected, pro- 

 visionally at least, places specifically, or even generically se- 

 parated, in which to range the separate pieces, than to hold 

 that they had all united in one anomalous genus ; though 

 such was actually the fact. And Agassiz, in erecting three 

 distinct genera out of the fragments of a single genus, has in 

 reality acted at once more prudently and more intelligently 

 than if he had avoided the error by rashly uniting parts which 

 in their separate state indicate no tie of connection. 



The cranial buckler of the Dipterus (fig. 20) was, like 

 that of the Diplopterus, of great beauty. In some of the 

 finest specimens we find the enamel ornately tatooed, within 

 the more strongly-marked divisions, by delicately traced lines, 

 waved and bent, as if upon the principle of Hogarth ; and 

 though the lateral plates are numerous and small, and defy 



