40 CEREBRAL DEVELOPMENT 



down to us in the fossil state are those of the ganoids of 

 the Lower Eed Sandstone ;* and in these lishes the tru(} 

 skull appears to have been as entirely a simple cartilaginous 

 box, as that of the placoids of either the Silurian period or 

 of the present time, or of those existing ganoids, the stur- 

 geons. In the Old Red genera Gheiracanthus and Diplacan- 

 thus, though the heads are frequently preserved as amorphous 

 masses of coloured water, we detect no trace of internal bone, 

 save perhaps in the gill-covers of the jfirst-named genus, which 

 were fringed by from eighteen to twenty minute osseous rays. 

 The cranium seems to have been covered, as in the shark 

 family, by skin, and the skin by minute shagreen-like scales; 

 and all of the interior cerebral framework which appears un- 

 derneath exists simply as faint impressions of an undivided 

 body, covered by what seem to be osseous points, — ^bonjr 

 molecules, it is probable, which encrusted the cartilage. The 

 jaws, in the better specimens, are also preserved in the same 

 doubtful style ; and this state of keeping is the common one 

 in deposits in which every true bone, however delicate, pre- 

 sents an outline as sharp as when it occupied its place in the 

 living animal. The dermal or skeleton of both genera, which 

 consisted, as has been shown (pages 28, 29), of shagreen-like 

 osseous scales and slender spines, both brilliantly enamelled, 

 IS preserved entire ; whereas the interior framework of the 

 head exists as mere point-speckled impressions j and the in- 

 ference appears unavoidable, that parts which so invariably 

 differ in their state of keeping now, must have essentially 

 differed in their substance originally. 



Now in the Gheiracanthus we detect the first faint indi- 

 cations of a peculiar arrangement of the dermal skeleton, in 



* That these remarks apply to fish of an earlier date, the placo-ganoids 

 of the Silurian, imparts to them an additional interest. The reader may 

 be very safe in regarding these two kinds of brain, the placoid and the 

 ganoid, as Silurian contemporaries.— L. M. 



