﻿INTRODUCTION. Xlll 



Fritsch *) venture to place it in that subclass without much hesi- 

 tation. Others (e. g. Huxley 2 and Traquair 3 ), however, prefer to 

 retain the arrangement originally suggested by Agassiz ; and the 

 current opinion seems to be that it is an annectent type between 

 the Elasmobranchs and the so-called Ganoids 4 . 



The Elasmob ranch characters of the Acanthodians were well 

 summarized by Huxley {op. cit.) no less than thirty years ago, and 

 all the statements still remain valid. The structure of the exo- 

 skeleton, the nature of the fin-spines, the absence of cranial bones, 

 the absence of membrane-bones connecting the pectoral arch with 

 the cranium, the exposed and well-separated condition of the gill- 

 clefts, and the course of the " lateral line " between the scales on 

 the trunk — all still remain typically Elasmobranch characters. It 

 may also be added that another point of resemblance between the 

 Acanthodians and ordinary Elasmobranchs is observable in the tail. 

 In the heterocercal tail of a Teleostomous fish, when the upper 

 lobe of the caudal fin disappears, it is invariably replaced by a 

 series of ridge-scales ; in the Elasmobranchs, on the other hand, 

 though the disappearance of the . upper caudal fin-lobe is frequent, 

 it is always absolute, and leaves no trace of the former presence of 

 the appendage in a modification of the squamation. The latter is 

 the case among the Acanthodians, of which none but completely 

 heterocercal types are known. 



The so-called " Ganoid characters " of the Acanthodians were also 

 enumerated by Huxley when discussing this group ; but, unlike the 

 previous series of statements, they have proved for the most part 

 untenable. As pointed out by Pander 5 and Traquair 6 , the resem- 

 blances between Cheirolepis and Acanthodians are merely superficial. 

 The ring of circumorbital plates, suggesting a comparison with 

 Palceoniscus, has lately been discovered by Newberry 7 in a Palaeozoic 



1 A. Fritsch, ' Fauna der Gaskohle,' vol. ii. (1889), p. 96. 



2 T. H. Huxley, Mem. Geol. Surv. dec. x. (1861), p. 38. 



3 R. H. Traquair, Trans. Eoy. Soc. Edinb. vol. xxx. (1881), p. 18; also Geol. 

 Mag. [3] vol. v. (1888), p. 511. A query is appended to the arrangement in 

 Proc. Roy. Soc. Edinb. vol. xvii. (1890), p. 387. 



4 K A. von Zittel, ' Handbuch der Palseontologie,' vol. iii. (1887), p. 165. 



5 C. H. Pander, ' Ueber die Saurodipterinen, Dendrodonten, Glyptolepiden, 

 und Cheirolepiden des devonischen Systems ' (1860), pp. 69-73, with plates. 



6 R. H. Traquair, Ann. Mag. Nat. Hist. [4] vol. xv. (1875), p. 240, pi. xvii. 



7 J. S. Newberry, Mon. U.S. Geol. Surv. no. xvi. (1889), p. 104 (" eye- 

 capsules "), pi. xliv. fig. 3 {Cladodus kepleri). 



