OF THE EARLIER VERTEBRATA- 61 



sentation of the most ancient brain-pan on which human eye 

 has yet looked, — as, in short, the type of cell in which, my 



Fi^. 23. 



LONGITUDINAL SECTION OF HEAD OF DIPTEEUS. 



riads of ages ago, in at least one genus, that mysterious sub- 

 stance was lodged on whose place and development so very 

 much in the scheme of creation was destined to depend. The 

 specimen from which the figure is taken was laid open late- 

 rally by chance exposure to the waves on the shores of Thurso ; 

 another specimen, cut longitudinally by the saw of the lapi- 

 dary, yields a similar section, but greatly more compressed in 

 the cavities; on which, of course, as unsupported hollows. 

 the compression to which the entire cranium had been ex- 

 posed chiefly acted. When the top and bottom of a box are 

 violently forced together, it is the empty space which the box 

 encloses that is annihilated in consequence of the violence. 



It is deserving of notice, that the analogies of the cranial 

 cavities in this ancient ganoid should point so directly on 

 the cranial cavities of that special ganoid of the present time 

 which unites a true skull of cartilage to a dermal skull of 

 osseous plates, — a circumstance strongly corroborative of tlie 

 general evidence, negative and positive, on which I have con- 

 cluded that the true skulls of the first ganoids were also car- 

 tilaginous. It is further worthy of observation, that in all 

 the sections of the cranium of Dipterus which I have yet 

 examined, the internal line is continuous, as in the placoids, 

 from nape to snout, and that the true skull presents no trace 

 of those cerebral vertebrae of which skulls are regarded by 

 Oken and his disciples as developments. Historically at 

 least, the progress of the ichthvic head seems to have been 



