1G6 THE PROGRESS OF DEGRADATION. 



that in some placoid genera, such as Scyllium Stellare, tlif^ 

 terminal portion of the fin is scarce less largely developed 

 above than below, and that in others, as in most of the r:i 

 family, the under lobe of the fin is wholly wanting. In tli. 

 sturgeon, — one of the few ganoids of the present time, — we 

 become sensible of a peculiar modification in this heterocer- 

 cal type of tail ; the lower lobe is, we find, composed, as in 

 Spinax and Scyllium, of rays exclusively ; while through the 

 centre of the upper lobe there runs an acutely angular patch 

 of lozenge-shaped plates, like that which runs through the cen- 

 tre of the double fins of Dipterusdin^ some of the Celacanths. 

 But while in the sharks the gradually diminishing vertebrae 

 stand out in bold relief, and form the thickest portion of the 

 tail, that which represents them in the sturgeon (the angular 

 patch) is slim and thin, — slimmer in the middle than even at 

 the sides ; — in part a consequence, no doubt, of the want, 

 in this fish, of solid vertebrae, but a consequence also of the 

 extreme attenuation of the nervous cord, in its prolongation 

 into the lobe of the fin. Further, the rays of the tail, — 

 its peculiarly ichthyic portion, which are purely mucoidal in 

 Spinax, Scyllium, and Cestracion, — have become osseous in 

 the sturgeon. Hhefish has set and become ^a;ec^, as cement 

 sets in a building, or colours are fixed by a mordant. And 

 it is worthy of special remark that, correspondent with the 

 peculiarly ichthyic development of tail in this fish, we find 

 the prevailing ichthyic displacement of the fore limbs. Again, 

 in the tail of Lepidosteus, another of the true ganoids which 

 still exist, the internal angle of the upper lobe wholly dis- 

 appears, and with the internal angle the prolongation of the 

 nervous cord. Still, however, it is what the tail of the 

 sturgeon would become were the angular patch to be oblite- 

 rated, and rays substituted instead ; — it is a tail set on awry. 

 And in this fish also we find the ichthyic displacement of 

 fore limb. One step more, and we arrive at the homocer- 



