52 



PISCES 



CLASS I 



Family 3. Pteraspidae. Smith Woodward. 



Dermal armour of head region completely fused into large plates. External layer of 

 each dermal plate forming an ornament of very fine, concentric, closely arranged ridges, 

 parallel with the outer margin ; middle layer with large polygonal cancellae. Dermal 

 sense organs well developed, arranged in canals traversing the middle layer of the 



shield and opening by a 

 double series of pores ex- 

 ternally. Ventral shield 

 simple. Tail presumably 

 heterocercal, and scales 

 of caudal region, when 

 preserved, numerous and 

 rhomboidal. Upper 

 Silurian and Lower 

 Devonian. 



Three genera of this 

 family are distinguished 

 according to the com- 

 plexity of the dorsal 

 shield. The simple 

 ventral shield, similar 

 in each form, was 

 originally mistaken for 

 dorsal armour, and 

 named Scapliaspis by 

 Lankester. 



I'ic. 101 



Pte 



raspis, 



Kner 



"A. /;. Ptt raspis rostrata, Ag. sp. Lower Old Red Sandstone (Cornstones) ; (PalaeoteiltMs, .Al'chaeo- 

 Heretbrdshire. .4, Dorsal shield, 1/2 na t- size, restored. B, Ventral jdiield f^+j,-,, t? 



(Scaphaspis lloydi, Ag. sp.), 2/3 nat. size (after Ray Lankester). C, Dorsal teUttllS, £. xCoemerj, 

 and ventral shields of Pteraspis, lateral aspect, restored from a specimen /f ip-g 101-103) Doi'- 

 found in Galicia (after Alth). *" it-i-i-ii 



sal shield arrowhead- 

 shaped, consisting of seven separately calcified plates — a large central disc ; 



■~— c 



Fig. 102. 



Pteraspis rostrata, Ag. sp. Piece 

 of shield, much magnified, a, Upper, 

 b, Middle, and c, Lower layer (after 

 Ray Lankester). 



Fig. 103. 



Pteraspis rostrata, Ag. sp. Vertical section of head 

 shield parallel with a longitudinal rib, highly mag- 

 nified (after Ray Lankester). 



a triangular rostral plate in front ; a median spine behind ; a pair of orbital 

 plates, completely enclosing the orbit on either side, and partially inserted 

 between the rostrum and central disc ; and a pair of lateral cornua, each 



