,g^. FORE-RUNNERS OF BACKBONED ANIMALS. 6oi 



In some genera of Cephalaspidians, as Aiichenaspis (Fig. 6) and 

 Didymaspis, a few of the anterior body-scales fuse together into a 

 continuous plate above and below, and that again is usually fixed to 

 the head-shield ; but even in such a case there is no appearance of a 

 shoulder girdle. The advance in this direction is greatest in the 

 genus Tyemataspis (Fig. 4), which exhibits no dividing line between 

 the original head-shield and the coalesced body-scales ; and the 

 armour thus named is further of special interest, because the small 

 bone between the eyes {orb.) is loose, not fixed, and because the 

 postorbital \2A\ey (p.o.v.) seems to be without an infilling plate. In 

 Tremataspis, also, the small antorbital organ [ant.) common to all 

 Cephalaspidians, is distinctly perforated, and the view has recently 

 been expressed by Rohon (6) that this corresponds with the special 

 sense-organ — " stirnorgan " — of Selenka (7). 



From the highly-specialised Tremataspis, we may pass to the 

 third great division of the Ostracoderms, of which the familiar 

 " winged-fish " {Pterichthys) of Agassiz and Hugh Miller may be 

 taken as type. These organisms have been most carefully investi- 



FiG. 10. — Restoration o! Cephalasfis uunchisoui, from speciniers discovered by G. H. Piper, Etq. 

 (From But. Miis. Catal. Foss. Fislies.) 



gated by Dr. R. H. Traquair (8), and the restorations of Pterichthys, 

 given in Figs. 7, a, b, are very slightly modified from his drawings* 

 The shield, it will be observed, is essentially similar to that of those 

 Cephalaspidians in which some body-segments are fused together ; 

 but the head is sharply separated from the trunk, and the armour, 

 instead of being simple, is composed of a number of symmetrically 

 arranged overlapping plates. It is also remarkable for the great 

 pair of pectoral appendages (ap.), which are hollow, moveabl}' arti- 

 culated, consist of many plates, and are once jointed (j.). The 

 small plate between the eyes (orb.) is loose, and marked inside with 

 a pit that seems to have lodged the rudimentary pineal eye ; a cheek- 

 plate on each side is moveable (at least in Pterichthys), and probably 

 formed the operculum (op.) ; while a pair of loose jaw-plates at the front 

 of the head below (Fig. y a, m.) are finely toothed on their posterior 

 border (at least in Bothriolepis). Of the precise nature of the nose, 

 mouth, and jaws, however, nothing is as yet known. Behind the 

 plated armour in Pterichthys, the tapering, scaly tail is often preserved, 

 and the median fins are arranged exactly as in Cephalaspis. 



The question now arises as to the true nature of the pectoral 

 appendages, which form so distinctive a feature of Pterichthys and its 

 allies, as compared with the two lower groups of Ostracoderms 



