SUB-CLASS II 



OSTRACODERMI 



51 



&m 



shire, they were commonly supposed to belong to sharks. The latter discovery 

 is coiisidered by Traquair to indicate some close relationship between the 

 Ostracophores and the primitive Selachii. 



Thelodus, Ag. {öoelolepis, Pachylepis, Thelolepis, Pander ; Cephalopteriis, 

 Powrie; Turinia, Traq.), (Fig. 99). Dermal tubercles small, quadrangular, 

 and nearly uniform. A small dorsal fin near base of 

 heterocercal tail. No enlarged ridge scales. T. parvidens, 

 Ag. ; detached tubercles in Ludlow Bonebed and Oesel 

 Limestone. T. scoticus, Traq. ; complete skeletons in 

 Upper Ludlow, Logan Water, Lanarkshire. T. pagei, fio. 99. 



Powrie sp. ; complete skeleton, Lower Old Ked Sand- Theiodns parvidens, Ag. 



rn-TT-n-n i- ^ ' DeiTTial tubercles, nat. size 



Stone, iurin Hill, J^ Oriarshire. (above) and enlarged. Upper 



Lanarkia, Traq. Dermal tubercles are small, pointed, 8i^™(L"diowBoneBed); 



hollow spines, not of uniform size. L. horrida, Traq., 



and other species represented by skeletons in the Upper Silurian Passage 



Beds of Birkenhead Burn and 

 Seggieholm, Lanarkshire. 



Family 2. Psammosteidae. 

 Traquair. 



Dermal armour of head region 

 more or less completely fused info large 

 plates. External layer of each dermal 

 plate forming a tubercular ornament. 

 Dermal sense organs not penetrating the 

 tissue of the armour. Devonian. 



Fsammmteus, Ag. (Placosteus, 

 Ag. ; Fsammolepis, Ag.). External 

 Ornament of very closely arranged, 

 rounded, or elongated tubercles 

 /> ^ which are usually crimped rpund 

 the margin. Orbit probably enclosed 

 in the shield. Paired spines (of un- 

 certain position) broad and tri- 

 angulär, with a large internal cavity 

 and Short base of insertion. Large 

 ridge scutes on the tail, ornamented 

 as the body shield and paired spines. 

 P. maeandrinus, Ag.,and P.paradoxus, 

 Ag., from Upper Devonian, Russia. 

 P. taylori, Traq., from Upper Old Red 

 Sandstone, Elgin. Other species from 

 the Lower Old Red Sandstone of 

 west of England, and from Upper 

 Devonian, Spitzbergen. 



Drepanaspis, Schlüter (Fig. 100). 



l, Postero-lateral plates; p.v.l, Postero-ventro-lateral Head reglOU COVered Wltn OUe large 

 l.iates ; r, Rostral plates ; x, Orbits. 1/4 (after Traquair). ^g^Jj^n plate, tWO pOStCrO - lateral 



plates, two rostral plates, and intervening small polygonal plates. Tail as in 

 Psammosteus. D. gemuendenensis, Schlüt., from Lower Devonian, Gemünden, Eifel. 



Drepanaspis gemuendencnsis, Schlüter. Restored out- 

 line of ventral aspect, surface ornament omitted. Lower 

 Devonian ; Gemünden, Eifel. m.v, Median ventral plate; 



