C. R. Eastman — Asterolepid Appendages. 141 



Art. XVIII. — Asterolepid Appendages ; by C. R. Eastman. 



The renewed interest which has become manifest in the 

 study of the earliest known fish-like vertebrates bids fair to 

 lead to a clearer understanding of the problems presented, if 

 not indeed to their immediate solution. One of the most 

 puzzling riddles is that concerning the derivation and homology 

 of the pectoral limbs, or "swimming appendages" of the 

 Asterolepidw, the only family of Ostracophores in which 

 such organs occur. Different interpretations of their nature have 

 led to a wide range of opinion in regard to the relations of 

 Ostracophores in general, some authors uniting them with fishes 

 proper and others separating them, some deriving them from 

 the Elasmobranch stem, some from the Crossopterygian, and 

 some even from Arachnids. It is still a mooted point whether 

 Ostracophores and Arthrodires are genetically related, some 

 favoring the retention and others the abolition of the group 

 Placodermata as originally proposed by M'Coy. It is upon 

 controverted questions of this nature that a study of Asterolepid 

 appendages may be expected to throw some light. 



A variety of opinions has been expressed concerning the prob- 

 able origin and homology of these organs. Thus, there may 

 be noted : first, the theory that they partake of the nature of 

 Arthropod appendages, on the assumption that Ostracophores 

 are descended from Arthropods ; secondly, the theory which ex- 

 plains Asterolepid limbs as the produced and jointed head-angles, 

 or " cornua," of forms like Cephalaspis; thirdly, the theory that 

 they are derived from a fixed spine attached to the body, simi- 

 lar to the spinous appendage of Acanthaspis / fourthly, the 

 theory that they are derived from the lobate Crossopterygian pec- 

 toral fin by a process of reduction and specialization ; and lastly, 

 the theory that they are independently derived. 



The first two of these hypotheses are evidently founded upon 

 complete misconceptions of the structure and position of Astero- 

 lepid limbs, and may be summarily dismissed. The third, 

 which explains them as having become evolved from a fixed 

 spine, presupposes anomalous, if not impossible conditions. 

 Neither can they be looked upon as modified pectoral fin-spines, 

 the remainder of the fin having become atrophied ; for no simi- 

 lar case of reduction is known amongst fishes. The fourth sug- 

 gestion, that these members are derived from specialization of 

 Crossopterygian pectoral fins, has recently been put forward by 

 Mr. C. Tate Regan,* and has not yet been discussed. The fore- 

 most objection that may be urged against this theory is that it 



* Regan, C. T., The Phylogeny of the Teleostomi (Ann. Mag. Nat. Hist, 

 ser. 7, vol. xiii, pp. 329-349), 1904. 



