OF THE ASTEROLEPIS. 



31 



Fig. 6. 



bours impinged upon it along these grooves to the extent of 

 about one-third its area, — and 

 that it impinged, in turn, to the 

 same extent, on the scales that 

 bordered on it posteriorly and 

 latero-posteriorly. Now, in the 

 Celacanth family (and on this spe- 

 cial point the foregoing remarks 

 are intended to bear), the scales, 

 which were generally of a round 

 or irregularly oval form (fig, 6, 

 b), overlapped each other to as 

 great an extent as in any of the 

 existing fishes of the Cycloid or 

 Ctenoid orders, — to as great an 

 extent, for instance, as in the carp, 

 salmon, or herring. In a slated 

 roof there is no part on which 

 the slates do not lie double, and 

 along the lower edge of each tier 

 they lie triple ; — there is more of 

 slate covered than of slate seen : 

 whereas in a tile-roof, the co- 

 vered portion is restricted to a 

 small strip running along the top 

 and one of the edges of each tile, and the tiles do not lie 

 double in more than the same degree in which the slates lie 

 triple. The scaly cover of the two genera of DipteriMus to 

 which I have referred was a cover on the tile-roof principle ; 

 and this is an exceedingly common characteristic of the scales 

 of the ganoids. The scaly cover of the Celacanths, on the 



* These scales, wliich occur in a detached state, in a stratified clay of 

 the Old Red Sandstone, near Cromarty, present for their size a large: 

 extent of cover than the scales of any other ganoid. 



Scales of Osteolepis Microlc 

 pidotus. 

 h. Scales of an undescribed spe- 

 cies of Glyptolepis* 



(The single scales mag. two dia- 

 meters, — the others nat. size.) 



