ANATOMY OF VERTEBRATES. 



197 



& 



B 



li 



N\ 



cuirass. In the composition of this armour, as defined by 

 sutures, not mucous grooves, may be ^7 



discerned the following plates : 5, 

 median ; 6, lateral ; 7, premedian ; 8, 

 prelateral; 9, rostral; 12, dor somedian; 

 14, postdor somedian ; is, sublateral; 20, 

 postventrolateral ; 22, preventrolateral ; 

 24, suborbital. 



The blank space between the neu- 

 ral, n, and haemal, h, spines of the 

 fossil endoskeleton indicates the posi- 

 tion of the soft ' notochord,' c, which 

 has been dissolved away. 



In the Pterichthys of the same geo- 

 logical formation, the helmet was 

 moveably articulated with the trunk- 

 buckler. 



In Cephalaspis the armour of the 

 head was shield-shaped, with the pos- 

 terior angles produced backward in a 

 pointed form. 



The fishes with enamelled dermal 

 bones in the form of plates, whether 

 coarticulated, fig. 127, or detached as in 

 the Sheat-fishes and Sturgeons, fig. 125, 

 dp, d s, are called ' placoganoid : ' c/^0^^{ 

 those in which they have the size, ^M^^^^ 

 form, and overlapping arrangement of o^^^^f 

 scales, fig. 126, are called ' lepidoganoid.' 

 The genera Polypterus and Lepidosteus 

 exceptionally exemplify the latter con- 

 dition of the dermoskeleton at the 

 present day : it was the rule with the 

 fishes of the mesozoic period, and 

 with those of the palaeozoic which 

 were not ' placoid ' or ' placoganoid.' 



In fig. 126, a indicates the outer 

 surface of parts of two series of the 

 rhomboidal ganoid scales of the extinct 

 Amblypterus : and b the inner surface 

 of two scales, showing the ridge pro- 

 duced at one end into a projecting 

 peg, which fits into a notch of the next Bnd<Hind exo-skeieton, coccosum. clvi. 



£ 



