50 



CEREBRAL DEVELOPMENT 



Fig. 19. 



perior frontal. Of the cartilaginous box which formed the 

 interior skull of either Osteolepis or Diflopterus, or, with but 

 one exception, of the interior skulls of any of their contem- 

 poraries, no trace, as I have said, has yet been detected. The 

 solitary exception in the case is, however, one of singular in- 

 terest. 



In a collection of miscellaneous fragments sent me by Mr 

 Dick from the rocks of Thurso, I de- 

 tected patches of palatal teeth ranged 

 in nearly the quadratures of circles, 

 and which radiated outwards from the 

 rectangular angle or centre (fig. 19, 

 h). And with the patches there oc- 

 curred plates exactly resembling the 

 barbed head of a dart (a), with which 

 I had been previously acquainted, 

 though I had failed to determine 

 their character or place. The excel- 

 lent state of keeping of some of Mr 

 Dick's specimens now enabled me to 

 trace the patches with the dart-head, 

 and several other plates, to a curious piece of palatal me- 

 chanism, ranged along the base of a ganoidal cranium, covered 

 externally by a brightly enamelled buckler, and to ascertain 



tegrating wear of the surf, — and exhibits, in consequence, the walls of the 

 brain-chamber lying underneath, with a narrow cavity or passage run- 

 ning downwards from the chamber towards the little central plate. And 

 along this passage, the prolongation of the brain, which terminated in 

 the pineal gland, seems to have descended. The reader will of course 

 see to what the evidence here actually amounts. A witness of credit 

 states that there once ran a certain prolongation of a certain organ, long 

 since reduced to dust, from one indicated point to another, and this in a 

 direction in which it had not been previously known that there existed a 

 passage for its transmission. An opportunity of observation occurs, how- 

 ever, — that furnished by the organism here figured, — and the required 

 passage is found running in the indicated direction. 



a. Palatal dart-head. 



b. Group of palatal teeth. 



