SUB-CLASS II 



OSTRACODERMI 



51 



Cephaloptertis, 

 quadrangular, 



Fic). 99. 



Thclodus ■parvidens, Ag. 

 Dermal tubercles, nat. size 

 (above) and enlarged . Upper 

 Silurian (Ludlow Bone Bed) ; 

 Ludlow. 



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, Fachylepis, 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. 



Lanarhia, 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 Passage 



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. 



Psmmnosteus, Ag. (Placosteus, 

 Ag. ; Psammolepis, 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). 



Fig. 100. 



Drepaiw,ipis gerimendenensis, Sclililter. Restored out- 

 line of ventral aspect, surface ornament omitted. Lower 



Devonian ; Gemiinden, Eif el. m.r, Median ventral plate ; _ _ - . n -.i i 



p.l, Postero-lateral plates; p.v.l, Postero-ventro-lateral Head reglOU COVered With Olie large 

 plates;.,-. Rostral plates;.. Orbits. V, (after Traquair). ^^^-^^ ^^^^^^ ^^^ pOSterO - lateral 



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

 Psammosteus. D. gemuendenensis, Schliit., from Lower Devonian, Gemiinden, Eif el. 



