ANATOMY OF VERTEBRATES, 



197 



cuirass. In the composition of this 

 sutures, not mucous grooves, may be 

 discerned the following plates : 5, 

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

 prelateral; 9, rostral:, 12, dorsomedian\ 

 14, postdorsomedian ; is, sublateral\ 20, 

 postventrolateral') 22, preventrolateral ; 

 24, suborbital. 



The blank space between the neu- 

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

 fossil endoskeleton indicates the posi- 

 tion of the soft f notochorcl,' 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, 

 d p, d s, are called f placoganoid : ' 

 those in which they have the size, 

 form, and overlapping arrangement of 

 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 paleozoic 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 

 Arriblypterus : 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 



armour, as defined by 



127 



J ^^ O ^-A r 



V'". a- f L, 



as. 



at*. 



& 



^ afe" 



':a> 



*r*. 



JR. 



u% 



Endo-and exo-skeleton, Coccosteus. CLVI. 



