THE PROGRESS OF DEGRADATION. 167 



cal or equal-lobed tail, wHch seems to attain to its extreme 

 type in those fishes in which, as in the perch and flounder, 

 the last vertebral joint, either very little or very abruptly di- 

 minished in size, expands into broad processes, without ho- 

 mologue in the higher animals, on which the caudal rays rest 

 as their bases. And in by much the larger proportion of 

 these fishes all the four limbs are slung round the neck ; — 

 they at once exhibit the homocercal tail in its broadest type, 

 and displacement of limb in its most extreme form. 



Now, in tracing the geologic history of the ichthyic tail, 

 we find these several steps or gradations from the heterocercal 

 to the homocercal represented by periods and formations.* 

 The Silurian periods may be regarded as representative of that 

 true heterocercal tail of the placoids exemplified in Spinax 

 (page 146, fig. 54) and Cestracion (page 151, fig. 55). The 

 whole caudal portion of this latter animal, commencing im- 

 mediately behind the ventrals, is, as becomes a true tail, slim, 

 when compared with its trunk ', the vertebrae are of very con- 

 siderable solidity ; the rays mucoidal ; and where the spinal 

 column runs into the terminal fin, it takes such an upward 

 turn as that which the horse-jockey imparts, by the process 

 oi nicking, to the tails of the hunter and the racehorse. And 

 with the heterocercal tail, so true in its homologies to the 

 tails of the higher vertebrata, we find associated, as has been 

 shown, the true homological position of the fore limbs. With 

 the commencement of the Old Red Sandstone the ganoidal 

 tail first presents itself ; and we become sensible of a change 

 in the structure of the attached fin, similar to that exempli- 

 fied in the caudal rays of the sturgeon. As shown by the ir- 

 regularly-angular patch of scales which in all the true Cela- 



* Since ganoids have been found as early as placoids, and even ear- 

 lier, the theory of these representative periods must be modified. How 

 tJie watter will ultimately stand as regards the Cambrian and the t<>ro 

 Silurians we of course do not know — L. M. 



