RAMBLES OF A GEOLOGIST. 435 



broader in proportion to its length than that of the Diplop- 

 terus, consisted of many more plates. I may here mention 

 that the frontal buckler of Diplopterus has not yet been 

 figured nor described ; whereas that of Dipterus, though un- 

 known as such, has been given to the world as the occipi- 

 tal covering of a supposed Cephalaspian, the Polyphractus. 

 Polyphractus is, however, in reality a synonym for Dipterus, 

 the one name being derived from a peculiarity of the ani- 

 mal's fins; the other, from the great number of its occipital 

 plates. There is no science founded on mere observation that 

 can be altogether free, in its earlier stages, from mistakes of 

 this character, mistakes to which the palaeontologist, how- 

 ever skilful, is peculiarly liable. The teeth of the two genera 

 were essentially different. Those of the Dipterus, exclusively 

 palatal, were blunt and squat, and ranged in two rectangular 

 patches;* while those of the Diplopterus bristled along its 

 jaws, and were slender and sharp. Their tails, too, though 

 both heterocercal, were diverse in their type. In each, an 

 angular strip of gradually-diminishing scales, a prolongation 

 of the scaly coat which protected the body, and which cover- 

 ed here a prolongation of the vertebral column, ran on to 

 the extreme termination of the upper lobe ; but there was in 

 the Diplopterus a greatly larger development of fin on the 

 superior or dorsal side of the scaly strip than on that of the 

 Dipterus. If the caudal fin of the Osteolepis be divided longi- 

 tudinally into six equal parts, it will be found that one of 

 these occurs on the upper side of the vertebral prolongation, 

 and five on the under ; in the caudal fin of the Diplopterus 



* I can entertain no doubt that the angular groupes of palatal teeth 

 figured by Agassiz and the Russian geologists as those of a supposed Placoid 

 termed the Ctenodus, are in reality groupes of the palatal teeth of Dip- 

 terus In some of my specimens the frontal buckler of Polyphractus is 

 connected with the gill-covers and scales of Dipterus, and bears in its 

 palate what cannot be distinguished from the teeth of Ctenodus. The three 

 genera resolve themselves into one. 



