GEOLOGICAL SURVEY IN SILURIAN ROCKS OF SOUTH OF SCOTLAND. 853 



No doubt this conclusion would be invalidated if it were proved that any 

 Pteraspidian possessed distinct Crossopterygian-like paired fins, like those indicated by 

 Claypole in his " attempted restoration " of Palceaspis Americana (v. p. 560). But this 

 is far from being the case. Judging from Prof. Claypole's sketches it does seem to me 

 that the resemblance of the objects in question to Crossopterygian fins is rather super- 

 ficial, and the author himself admits that " no specimen (as aforesaid) shows this organ 

 in position." Prof. Jaekel of Berlin (xiii. p. 467) has expressed the opinion that the 

 supposed fins are isolated portions of the dermal armature, and probably scales* A 

 different view, but equally decisive against these objects being fins, has been put 

 forward by Dr Bashford Dean of New York (viii. p. 71, footnote) in the following 

 words : — 



" The presence of paired fins in Palseaspis, as determined by Claypole, has not been confirmed. The 

 present writer, to whom the type specimens were kindly shown by their describer, must regard these 

 structures as elasmobrauchian (Chimaeroid?) spines, in crushed condition, accidentally associated with the 

 head region of the fossil." 



So with due apologies to Prof. Claypole, we cannot accept the occurrence of distinct 

 paired fins in the Pteraspidae as an ascertained fact. 



The Heterostraci. 



If the Pteraspidae are related to the Drepanaspidae, then it follows, by the same line 

 of reasoning, that they are also related to the Psammosteidse, and finally to the 

 Coelolepidae. The conception of the Heterostraci is, therefore, widened by the addition 

 of three families, showing almost every gradation from the shark-like Thelodus, with 

 its shagreen-covered skin and lappet-like pectoral fin-folds, to Pteraspis, which, with its 

 box-like carapace, composed of a limited number of sculptured plates, its scaly tail, 

 presents us with as good a type of the so-called " Placoderm " or " Panzerfisch " as any 

 palaeozoic creature which has ever been brought under that designation. 



What characters are now to be considered as common to the members of this order ? 



They all have this in common, that the microscopic structure of the dermal hard 

 parts (unknown, however, in Drepanaspidse) is either that of dentine, or at least of a 

 substance partaking more of the nature of dentine than of bone.-j- In none has any 

 internal skeleton been found, or any distinct jaws or teeth. The eyes, where their 

 position has been observed, are situated on the outer edge on each side of the anterior 

 part of the carapace. Those whose external form is sufficiently known have all a 

 strongly heterocercal caudal, but no other median fins. 



* "Nach Alledem glaube ich mit voller Sicherheit annehmen zu miissen, dass die fur Flossen gehaltenen 

 Skeletstiicke nichts anderes als isolirte Hautpanzertheile und zwar Schuppen des Fisches sind." 



t Schmidt (xxxi. p. 16-17) described and figured the occurrence of lacunae in the ridges of the Pteraspis-shield, 

 but this was denied by Lankester, and finally disproved by Rohon (xxvii. p. 74-75). As regards Psammosteus the 

 last named author maintains the occurrence, in the lowest layer of the shield of simple spindle-shaped bone cells which, 

 however, "weisen fast gar keine Primitivrohrchen auf" (xxviii. p. 70-71). These I have yet not seen, but Prof. 

 Rohon says they are only visible in especially well preserved examples. 



