INTRODUCTION. Xlll 
Eritsch 1 ) 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 he that it is an annectent type between 
the Elasmobranchs and the so-called Ganoids 4 . 
The Elasmohranch 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 Elasmohranch 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 Cheirolejpis 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. Eritsch, ‘ 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. Roy. 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 Palaeontologie,’ vol. iii. (1887), p. 165. 
5 C. H. Pander, ‘ Libber 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. Newberrv, Mon. E.S. Geol. Surv. no. xvi. (1889), p. 104 ("eye- 
*/ ' 
capsules ”), pi. xliv. fig. 3 ( Cladodus kepleri). 
