466 Notices of Memoirs— Dr. R. H. Traqiiair's Address. 



have here a strange creature whose shape entirely reminds us of 

 that of Tkelodus, having the same flat broad anterior part, bluntly 

 rounded in front and angulated behind, to which is appended 

 a narrow tail ending in a heterocercal caudal fin, which is, however, 

 scarcely bilobate. But here the dermal covering, instead of con- 

 sisting of separate scales or spinelets, shows a close carapace of 

 hard bony plates, of which two are especially large and prominent 

 — the median dorsal and the median ventral — other large ones being 

 placed around the margins, while the intervening space is occupied 

 by a mosaic of small polygonal pieces. The position of the mouth, 

 a transverse slit, is seen just at the anterior margin ; it is bounded 

 behind by a median mentum or chin-plate, but no jaws properly so 

 called are visible, nor are there any teeth. Then on each margin 

 near the front of the head is a small round pit, exactly in the 

 position of the dark spot seen in some examples of Thelodus, which, 

 if not an orbit, must indicate the position of some organ of sense. 

 Again, the tail is covered with scales after the manner of a ' ganoid ' 

 fish, being rhombic on the sides, but assuming the form of long 

 deeply imbricating fulcra on the dorsal and ventral margins. The 

 position of the branchial opening, or openings, has not yet been 

 definitely ascertained. 



All these plates are closely covered with stellate tubercles, and 

 we cannot escape from the conclusion that they are formed by the 

 fusion of small shagreen bodies like those of Thelodus, and united to 

 bony matter developed in a deeper layer of the skin. 



If the angular lateral flaps of Thelodus represent pectoral fins, 

 then we would have the exceedingly strange phenomenon of such 

 structures becoming functionally useless by enclosure in hard un- 

 yielding plates, though still influencing the general outline of the 

 fish. Be that as it may, can we doubt that in Drepanaspis we have 

 a form derived by specialization from a Ccelolepid ancestor ? 



This Drepanaspis throws likewise a much desired light on the 

 fragmentary Devonian remains known since Agassiz's time as Fsam- 

 mosteus. These consist of large plates and fragments of plates, 

 composed of vaso-dentine, and sculptured externally by minute 

 closely-set stellate tubercles, exactly resembling the scales of some 

 species of Thelodus. These tubercles are also frequently arranged in 

 small polygonal areas, reminding us exactly of the small polygonal 

 plates of Drepanaspis, and, like them, often having a specially large 

 tubercle in the centre. That Psammosteus had an ancestry similar to 

 that of Drepanaspis can also hardly be doubted. 



Finally, in the well-known Pteraspis of the Upper Silurian and 

 Lower Devonian formations we have a creature which also has the 

 head and anterior part of the body enveloped in a carapace, to which 

 a tail covered with rhombic scales is appended behind, and, though 

 the caudal fin has never been properly seen, such remains of it as 

 have occurred distinctly indicate that it was heterocercal in its 

 contour. The plates of the carapace have a striking resemblance 

 in general arrangement to those of Drepanaspis, though the small 

 polygonal pieces have disappeared, and there is a prominent pointed 



