THE PROGRESS OF DEGRADATION-. 169 



angle of the tail of tlie sturgeon : the lozenge-shaped scales 

 run in acutely angular patches through their upper lobes ; but 

 such is their extreme flatness, as shown by the disposition of 

 the enamelled covering, that it appears exceedingly doubtful 

 whether any vertebral column ran beneath ; — they seem but 

 to have covered greatly diminished prolongations of the spinal 

 cord. In the base of the Secondary division, — another long 

 stage towards the existing state of things, — we find, with the 

 homocercal tail, which now appears for the first time, nume- 

 rous tails like that of the Lepidosteus (fig. 57), of an inter- 

 mediate type ; — they are rather tails set on awry than truly 

 heterocercal. The diminished cord has disappeared from 



Fig. 57. 



TAIL OF LEPIDOSTEUS OSSEUS. 



among the fin rays. In the numerous Lepidoid genus, and 

 the genera Semionotus and Tetragonolepis, — all ganoidal fishes 

 of the Secondary period, — this intermediate style is very 

 marked ; while in their contemporaries of the genera Urceus^ 

 Microdon, and Pycnodus^ we find the earliest examples of 

 true homocercal tails. And in the ctenoids and cycloids of 

 the Chalk the homocercal tail receives its fullest development 

 It finds bases for its rays in broad non-homological processes, 

 that spread out behind abruptly-terminating vertebrae (fig. 

 58), in the same period in which, by a strange process of de- 



