C. R. Eastman — Aster olepid Appendages. 143 



have become lost through atrophy, or else they were never 

 developed except amongst Antiarcha; and if the latter be true, 

 it is easy to see that Asterolepid appendages are unrelated to 

 the fins of fishes, since they originated in a different way and 

 are constructed upon a different plan. They may be regarded 

 with much probability as having developed from a muscular 

 flap, or integumentary extension of the body, being of kindred 

 nature with tactile or clasping organs, or with the frontal spines 

 of Chimgeroids. The fact that one of the dermal plates is pierced 

 and otherwise modified for their attachment would seem to 

 indicate that pari passu with the development of body-armour, 

 the paired muscular extensions also became encased in plates. 

 But sphinx-like though the problem be as to how and when these 

 structures originated, the evidence appears tolerably certain 

 that they have not been derived from the fins of Pisces proper. 



A word may be said in regard to the assumed " close relation- 

 ship of the Coccosteidm and Asterolepidce, " these families being 

 representative of groups which are united by Mr. Regan, 

 together with the Osteostraci, in a single order of Teleostomi. 

 We regard as abortive this author's comparison of Coccosteus 

 with Pterichthys, which leads hiin to the conclusion that " the 

 arrangement of the bones of the head, and especially that of the 

 dermal plates of the body, can easily be reduced to a common 

 plan"; and it is manifestly untrue that "in the arrangement of 

 the bones of the cranial roof Coccosteus is almost a typical 

 Crossopterygian. " -- A remote superficial resemblance there 

 may be, in that certain plates are symmetrically disposed with 

 reference to the median line, but no real homology can be 

 claimed to exist between the cranial elements of Coccosteus and 

 those of Crossopterygians and Stegocephalians. It is impossible 

 to insist too strongly that the jaw-parts of Coccosteids are totally 

 distinct from those of fishes proper, although a parallel exists 

 between them and the dental plates of Palseozoic Chimseroids. 

 Both the upper and lower jaw of Arthrodires consist of purely 

 dermal ossifications, and teeth, properly speaking, are absent. 

 As for the serrations which sometimes occur along the cutting 

 margin in one or both jaws, and function as teeth, these are 

 not structurally differentiated from the supporting bony tissue. 

 Moreover, the lower dental plate ("gnathal" of Dean), instead 

 of being articulated to the cranium, is suspended freely in the 

 soft parts, similarly as in Chimgeroids. There is no evidence 

 of pectoral fins amongst Arthrodires, and the lateral spine which 

 is attached to the ventral armour evidently has nothing in com- 

 mon with the pectoral limb of Asterolepids. 



The axial skeleton of Coccosteus, as depicted by Jsekel* with 



*Jaekel, 0., Ueber Coccosteus und die Beurtheikmg der Placodermen 



(Sitzungsber. Gesell. Naturforsch. Freunde, p. Ill), 1902. Ueber die 



Kuderorgane der Placodermen (ibid. pp. 178-181), 1893. 



