772 REPORT— 1900. 



creature whose shape entirely reminds us of that of TheloduSj 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 consisting 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 mental 

 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 darli 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 assum- 

 ing 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 lilce 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 should 

 have the exceedingly strange phenomenon of such structures becoming functionally 

 useless by enclosure in hard unyielding 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 specialisation from a Crelolepid ancestor ? 



This Drepianaspis throws likewise a much-desired light on the fragmentary 

 Devonian remains known since Agassiz's time as Psammosteus. 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 Psam- 

 mosteus 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 rostrum in front of the mouth ; and 

 it is to be noted that the small round apertures usually supposed to be orbits are 

 in a position quite analogous to that of the sensory pits in Drepanaspis. The 

 plates of the carapace of Pteraspis are not, however, tuberculated, but orna- 

 mented by fine close parallel ridges, the microscopic structure of which, along with 

 their frequent lateral crenulation, leaves no doubt in our minds that they have 

 been formed by the running together in lines of TIielodus-\\k& shagreen grains. 

 An aperture supposed to be branchial is seen on the plate forming the posterior 

 angle of the carapace on each side. 



Until these recent discoveries concerning the Coelolepidse and Drepanaspidte, 

 Pteraspis and its allies, Cyathaspis and Palceaspis, constituted the only family 

 included in the order Heterostraci of the sub-class Ostracodermi, distinguished, as 

 shown by Lankester, by the absence of bone lacunte in the microscopic structure 

 of their plates. It is now, however, clear that we can trace them back to an 

 ancestral family in which the external dermal armature was still iu the generalised 

 form of separate shagreen grains or spinelets. 



