850 DR RAMSAY H. TRAQUAIR ON FOSSIL FISHES COLLECTED BY THE 



des Ptcraspis vollstandig iiberfliissig, dass aber die leistenartigen Erhabenheiten der 

 Schildoberflache vom Pteraspis aus der Verschmelzung zablreicber Placoidschuppen 

 hervorgegangen sind davon kann man sicb obne Riicksicht auf den histiologisehen Bau 

 audi bei macroscopischer Betracbtung iiberzeugen." 



However, in the second part of his researches on the Silurian Fishes of Oesel (xxviii. 

 p. 105), the same author proposes an alternative and opposite view of the case — 

 namely, that the ridges of the Pteraspidse might have been the most primitive condition 

 of the dermal skeleton of the Vertebrata. out of which, by differentiation, the dermal 

 denticles (placoid scales) of the Selachii, as well as their modifications in the Ganoids, 

 Teleostei, &c, have arisen. Again, to quote his words : — 



" Indessen konnte ebensogut die gegentheilige Ansicht gelten, d. h. die Streifchen 

 und Plattchen der Pteraspiden konnten den ursprunglichen Zustand des Hautskelets 

 der Vertebraten bilden, so dass also aus den Streifen und Plattchen der Pteraspiden 

 durch Differenzirung die Hautzahnchen (Placoidschuppen) der Selachier gleichwie die 

 entsprechenden Modificationen bei deren Descendenten (Ganoiden, Teleostiern, 

 Amphibien, &c), entstanden waren. Demgemass wtirden die langlichen Streifen, 

 Leistchen oder Plattchen der Pteraspiden als die auf der niedersten Entwickelungstufe 

 befindlichen Hartgebilde an der Korperoberflache bei den Vertebraten darbieten." 



It seems to me that, on this occasion at least, second thoughts have not proved the 

 best, for to suppose that the ridges of so specialised a structure as a dermal plate of 

 Pteraspis are likely to be more archseic in character than the simple shagreen-bodies 

 of the Ccelolepidse formed round a simple papilla, seems to me to be indeed rather like 

 putting the cart before the horse. But Prof. Rohon does not seem to insist very 

 strongly on his new theory, as may be seen from the remarks which immediately follow 

 his enunciation of it. It is, moreover, interesting to see that in comparing the micro- 

 scopic structure of the ridges of Pteraspis with that of placoid scales, he makes in this 

 paper special reference to the Coelolepidae : — " da die Streifen und Plattchen der 

 Pteraspiden die gleiche microscopischc Structur wie die den recenten Placoidschuppen 

 gegentiber als einfachere und altere Hautzahnchen erkannten Ccelolepiden aufweisen 

 (ib., p. 106). 



Much remains still to be learned about the Pteraspidse, but the configuration and 

 structure of the carapace which covers the head and anterior part of the body in the 

 type genus Pteraspis is pretty well known through the researches of Huxley, 

 Lankester, Alth, and others. 



The head and anterior part of the body of Pteraspis is enclosed in a carapace, the 

 dorsal part of which shows evidence of having been originally composed of at least 

 seven distinct plates, though in these fossils, as we usually find them, the pieces are not 

 actually separate. Text-figure 6 shows the arrangement of the parts of the do 

 surface of the carapace, while the accompanying text-figure 7 is a restoration of the 

 entire fish slightly altered from the figure given by Smith Woodward in the second 

 volume of his Catalogue, p. 161. 



