52 



PISCES 



Family 3. Pteraspidae. Smitli Woochvard. 



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 

 heterocerccd, 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 doi"sal 

 shield. The simple 

 ventral shield, similar 

 in each form, was 

 originally mistaken for 

 dorsal armour, and 

 named Scaphasjns by 

 Lankester. 



Pteraspis, Kner 

 (Palaeoteuthis, Archaeo- 

 teuthis, F. Koemer), 

 (Figs. 101-103). Dor- 

 sal shield arrowhead- 

 large central disc ; 



,M' 



Fig. 101. 



^1, B, rtpnispis rostrata, Ay. sp. Lower Old Red Sandstone (Cornstones) ; 

 Heret'oidsliire. A, Dorsal shield, 1/2 nat. size, restored. B, Ventral shield 

 (Svapho.^'iiis lloi/dt, Ag. sp.), -/s nat. size (after Ray Lankester). C, Dorsal 

 and veiitial shields of Ptemspis, lateral aspect, restored from a specimen 

 found in Galicia (after AUh). | . 



shaped, consisting of seven separately calcified plates — a 



Fii 



Pteraspis rostrata, Ag. sp. Piece 

 of shield , much magnified, a. Upper, 

 '1, Sliddle, and o. Lower layer (after 

 Ray Lankester). 



PIfrasjm rogtrata, Ag. sp. Vertical section of head 

 shield parallel with a longitudinal rib, highly niag- 

 nilied (after Ray Laiikestei-). 



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 



