THE PLACOID BRAIN. 1-17 



in at least the earlier geologic periods, when they were greatly 

 younger as a class, they did not betray. 



In illustration of this view, I would fain recommend to the 

 reader a simple experiment. Let him procure the tail of a 

 common dog-fish (fig. 54 a), and, cutting it across about half 

 an inch above where the caudal fin begins, let him boil it 

 smartly for about half an hour. He will first see it swell, and 

 then burst, all around those thinner parts of the fin that are 

 traversed by the caudal rays, — wholly mucoidal, as shown by 

 this test, in their texture, and which yield to the boiling water, 

 as if formed of isinglass. They finally dissolve, and drop 

 away, with the surrounding cuticular integument ; and then 

 there only remains, as the insoluble framework of the whole, 

 the bodies of the vertebrse, with their neural and hcemal pro- 

 cesses. The tail has now lost much of its ichthyic character, 

 and has acquired, instead, a considerable degree of resemblance 

 to the reptilian tail, as exemplified in the saurians. I have 

 introduced into the woodcut, for the purpose of comparison, 

 the tail of the ichthyosaurus (b). It consists, like the other, 

 of a series of gradually diminishing vertebrae, and must have 

 also supported, says Professor Owen, a propelling fin, placed 

 vertically, as in the shark, which, however, from its perish- 

 able nature, has in every instance disappeared in the earth, 

 as that of the dog-fish disappears in the boiling water. It 

 will be seen that its processes are comparatively smaller than 

 those of the fish, and that the bodies of its vertebrae are 

 shorter and bulkier ; but there is at least a general corre- 

 spondence of the parts ; and were the tail of the crocodile, 

 of which the vertebral bodies are slender and the processes 

 large, to be substituted for that of the enaliosaur here, the 

 correspondence would be more marked still. After thus (h- 

 veloping the tail of the reptile out of that of the fish, — as the 

 Irish magician of the tale developed young ladies out of old 

 women, — simply by boiling, let the reader proceed to a second 



