62 



OLD RED SANDSTONE FISHES. 



l^lectognaths, the Cephalaspida may have had the aponeurotic part of their exo-skeleton 

 separated wholly or in parts from the upper layers of the cutis vera, which may or may not 

 have existed in a calcified condition. The series of scales or bones along the body of 

 Cephalaspis — so strongly recalling the cinctures of Callichthys (which has a complete endo- 

 skeleton) — are, probably, morphologically of the same nature as those structures, but ante- 

 riorly I have not been able to detect any modification of the flanking ' scales' in Ceplialaspis 

 in the form of clavicular bones. The very peculiar pectoral fins of Cephalaspia are unlike 

 those of Ganoids or Teleosteans ; their broad membranous character is most nearly repre- 

 sented in Sharks ; but just as the character of their skeleton does not render it inqiossiUe 

 that they were Elasmobranchiate, nor point distinctively to either Ganoids, Siluroids, 

 Lophobranchs, or Plectognaths as their modern congeners, so do the fins fail to give any 

 decisive indication, for have we not classed among Ganoids Holopfi/cJiius, Polyptems, 

 Amia, and Sturgeon ? 



It is best, then, to let the group Cephalaqnda stand alone,^ since they present a 

 combination of characters which is not inconsistent with any one of the recent types of 

 internal organization presented by Eish, whilst agreeing with no living type of skeleton 

 entirely. When their position in time is considered, it becomes very probable that they 

 possessed an internal skeleton and viscera of more general character than any of the higher 

 living Fishes now present to us, which would justify their distinct position. The solid 

 character of the aponeurotic skeleton in the body of CejjJialasjns and its vertical segmen- 

 tation, together with the non-calcification or the separation of the more superficial parts of 

 the integument and of the cartilaginous endo-skeleton, may be considered as indicating a 

 generalised condition of structure, in which the splint-skeleton was so far dominant as to 

 take on that self-repetition which is, in other cases, known to us as vertebral segmen- 

 tation. 



The Heterostraci are associated at present with the Osteostraci because they are 

 found in the same beds, because they have, like CejjJmlasjjisy a large head-shield, and because 

 there is nothing else with which to associate them. There is at present no evidence that 

 the body and fins of Pteraspis and its allies were like those of Cephalaspis, and the 

 shields are not so closely similar in plan, much less in histological structure, as to warrant 

 any inference of similarity in other parts. Especially it is to be noted that, unless the 

 apertures considered as orbits in Pteraspis represent the ant orbital fossae or olfactory 

 organ of CepihaJaspis, there is a total absence of such fossEe in the Pteraspidian shield, 

 whilst the alternative gives the absence of orbits, either of which is an important diff'erence. 



The discovery of the bodies of Heterostraci will be a most welcome addition to 

 science, and for this we wait. 



1 Professor Huxley's last suggestion on this matter ('Academy,' No. 2, p. 42) is that the CephalaspidcB 

 possibly connect the Monorhina (Lampreys) with the Sturgeons among the Amphirhine ; but why should 

 their exo-skeleton have so much importance attached to it as to indicate a leaning towards Ganoids rather 

 than Sharks ? Professor Huxley even speaks of the Cephalaspidce as Ganoids simply, which seems to be 

 taking too much on 'faith,' as regards their unknown viscera. 



