SUB-CLASS II 



OSTRACODERMI 



51 



Cephaloptems, 

 quadrangular, 



Fig. 99. 



TTielodus parvidens, Ag. 

 Dermal foibercles, nat. size 

 (above) and enlarged. [Tpper 



Silurian (Ludlow Bone Bed); 

 Ludlow. 



Passage 



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



is considered by Traquair to indicate some close relationship between the 



Ostracophores and the primitive Selachii. 



Thelodus, Ag. (Coelolepis, Pachylepis, Thelolepis, Pander 



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



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, 



Powrie sp. ; complete skeleton, Lower Old Red Sand 



stone, Turin Hill, Forfarshire. 



LanarMa, Traq. Dermal tubercles are small, pointed 



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



and other species represented by skeletons in the Upper Silurian 



Beds of Birkenhead Burn and 

 Seggieholm, Lanarkshire. 



Family 2. Psammosteidae. 

 Traquair. 



Dermal armour of head region 

 more or less completely fused into large 

 plates. External layer of each dermal 

 plate forming a tubercular ornament. 

 Dermal sense organs not penetrating the 

 tissue of the armour. Devonian. 



Psammosteus, Ag. (Placosteus, 

 Ag. ; Pscmmolepis, Ag.). External 

 ornament of very closely arranged, 

 rounded, or elongated tubercles 

 which are usually crimped round 

 the margin. Orbit probably enclosed 

 in the shield. Paired spines (of un- 

 certain position) broad and tri- 

 angular, 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, Schliiter (Fig. 100). 

 Head region covered with one large 

 median plate, two postero - lateral 

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

 Psammosteus. D. gnu ut ndenensis, Schliit,, from Lower Devonian, Gemunden, Eifel. 



Fig. loo 



Drepanaspis gemuendenensis, Schliiter. Restored out- 

 line of ventral aspect, surface ornament omitted. Lower 

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

 p.l, Postero-lateral plates; p.v.l, Postero-ventro-lateral 

 plates; r, Rostral plates ; x, Orbits. 1/4 (after Traquair). 



