G 1 'RA CA NTHIDA E 



193 



apparently allied to the Acanthodii [507]. The body of Gyracanthides 

 seems to have been broad, short, and somewhat depressed ; the tail well- 

 developed and heterocercal. An 

 anal and a dorsal fin are found ; 

 each provided with a strong anterior 

 spine, from which the membrane 

 stretched back along the body as in 

 Acantliodians. Very large pectoral 

 and smaller pelvic spines indicate 

 the paired fins ; a groove along the 

 hind edge of these spines probably 

 marks the attachment of the fin- 

 web, which, however, is unknown 

 (Fig. 165). The head, trunk, tail, 

 and fins are all covered with 

 closely fitting quadrangular scales 

 of small and nearly uniform size. 

 Unfortunately the minute structure 

 of these scales is still unknown ; 

 but there is said to be a small 

 pulp-cavity, which would point to 

 their being flattened denticles, rather 

 than true Acanthodian scales [507]. 

 The fin -spines, composed of vaso- 

 dentine, have a hollow base, and 

 are superficially ornamented with 

 grooves and tubercles. Neither 

 endoskeleton, nor teeth, nor circum- 

 orbital plates have been found ; 

 but there are " two pairs of hollow, 

 broad, triangular, free spines, of 

 fibrous texture, fixed near the in- 

 sertion of the pectoral fin-spines " ; 

 these also have a tuberculated or- 

 namentation on one surface. 



pv.sp. 



Fig. 165. 



Gyracanthus Murrtiyi, A. S. W. ; Carboni- 

 ferous, Victoria ; restored. (After A. Smith 

 Woodward.) Ventral view, with the tail twisted 

 The systematic position OI the so as to show the dorsal fin. a, anterior, and 

 avrapintliirlqp i<s Viv nn mpars rlpar £' l )0sterior paired body-spines; a./, anal, r.f, 



Lfyracantmciae is uy no means cieai . c .. uulal> and dfi dorsal nn . pspi pec t rai spine ; 

 While resembling the Acanthodii in pr-*p, pelvic spine. 

 many characters [50 7 ], these interest- 

 ing fossils show some affinity to the Ostracodermi (more especially the 

 Pteraspidomorphi) in the structure of the dermal spines and plates. 

 Possibly they form a connecting-link between these two groups, and will 

 afford a valuable clue to the real position of the Acanthodii. 



Gyracanthus, Ag. (Fig. 165); Lower Devonian, Canada; Carboni- 

 ferous, Europe, N. America. Oracanthus, Ag. ; Carboniferous, Europe, 

 N. America. 



15 



