GEOLOGICAL SURVEY IN SILURIAN ROCKS OF SOUTH OF SCOTLAND. 849 



the microscopic structure of the hard parts of Drepanaspis can be investigated and 

 more also is known regarding the configuration of Psammosteus and the arrangement of 

 its plates. 



But as regards Oracanthus, I must now abandon all idea of its being related to 

 Psammosteus, retaining it indeed as a veritable Selachian. Certainly it is so, if the 

 carboniferous fish which I named and described as Oracanthus armigerus (xxxii. p. 86) 

 has anything at all to do with the genus in which I placed it. The position of the 

 spines of 0. armigerus is at the back of the head, one on each side, like the cornua 

 of Cephalaspis, but the dentition is cochliodont, and the creature is evidently closely 

 allied to Menaspis armata, Ewald, from the German Kupferschiefer. 



Meanwhile the problem of the " armoured sharks " is, I think, solved, by our ceasing 

 to consider them, properly speaking, as sharks at all. and transferring them to the 

 Heterostraci. This is in accordance with views already expressed by Dr 0. M. Reis 

 of Munich, as we shall presently see. 



The Pteraspid^e. 



The Pteraspidse, the only family which by common consent has hitherto been 

 included in the Heterostraci of Lankester (Aspidorhini, Rohon), have, like the Psam- 

 mosteidse, no bone lacunae in the substance of their dermal plates, but the middle layer, 

 instead of consisting altogether of a dense reticulation of Haversian canals, forms in its 

 lower part at least a stratum of polygonal or prismatic cavities, which is the cancellated 

 layer. The outer layer consists of dentine, or kosmine, with fine arborescent tufts of 

 minute tubules. The external sculpture consists of fine concentric and sub-parallel 

 ridges and grooves, but the ridges, as Prof. Lankester observes, "are usually 

 crenated at the margins, and give the notion in some species of a linear series of 

 minute tubercles fused together." Further on he also observes : — " Indeed each of the 

 sections of the ridges recalls very strongly the structure of a tooth or of a dermal 

 defence of a placoid fish" (xiv. pp. 11-12). 



In fact the microscopic appearance of the ridges when seen cut in transverse section 

 at once recalls the structure of the tubercles of Psammosteus, or of the crowns of the 

 scales of Theloolus, and I have no doubt that we have the explanation of their origin 

 in the idea of the fusion of Ccelolepid dermal tubercles or shagreen-bodies in linear 

 order. That these ridges originated by the fusion of " placoid " scales of some sort at 

 least was strongly advocated by Rohon in the first part of his monograph on the 

 Upper Silurian Fishes of Oesel (xxvii. p. 75), where he says : — 



" Ich muss Prof. Ray Lankester beistimmen, wenn er in dem Bau der ausseren 

 oder Leistenschicht des Pteraspis- Schildes die Structur der Placoidschuppen erblickt 

 und diese Schicht aus demselben Grunde auf die Placoidschuppen zuruckfiihrt. Wird 

 an dieser vollkommen richtigen Anschauung festgehalten, so ist die Existenz der 

 Knochenzellen wie ganz treffend Ray Lankester hervorhebt, fur die Wirbelthiernatur 



