﻿XIV INTRODUCTION. 



fish which all will agree is a typical Elasmobranch. The " production 

 of the pectoral arch into long baekwardly directed processes in Dipla- 

 canthus" leading to a comparison with the Siluroids, is due merely 

 to a pair of spinous fin-rays, which have no known analogues either 

 among Elasmobranchs or Teleostomes. The " great spines articu- 

 lated with the pectoral arch " cannot be regarded as of much signi- 

 ficance. The so-called " oral tentacles " are endoskeletal structures, 

 and probably represent the ceratohyal bones with their appended 

 rays. Finally, the contention that the Acanthodii may be a degene- 

 rate branch of the " ganoids " that has followed and even descended 

 beneath the Chondrostean Polyodontidas, seems as destitute of 

 philosophical basis as the contrary supposition that they form an 

 Elasmobranch type on the verge of entering the Teleostomi. 



According to all reliable observations, when a bony squamation 

 degenerates, it is never accompanied by a simultaneous develop- 

 ment of the insignificant surface-layer of cosmine and vascular 

 dentine, but becomes replaced by a calcified tissue of thin lamellae. 

 It is thus contrary to widely-established principles to suppose that 

 the order under consideration has developed from fishes with an 

 osseous exoskeleton. On the other hand, the most typical of the 

 early Teleostomi have archipterygial paired limbs, and hence cannot 

 have been derived from the Acanthodii, which possess extremely 

 specialized and abbreviated paired fins. The only alternative theory 

 by which any connection whatever can be admitted between the 

 two groups, seems to be the ordinary resource of a modern taxono- 

 mist in difficulties — the polyphyletic origin of the higher type. 



Ear from resorting to this solution of the problem, we prefer to 

 interpret the anatomical characters of the Acanthodian fishes as 

 proving that they occupy the same position in the Elasmobranch 

 phylum that is held at the present day by the Actinopterygians in 

 that of Teleostomi. Their abbreviate fins, degenerate dentition, and 

 the partial development of membrane-calcifications 1 9 indicate their 

 comparatively advanced status in whatever subclass they may be 

 placed ; and in the present condition of knowledge, it seems best to 

 regard them as the culminating series of the Elasmobranchii at the 

 time when this subclass was one of the dominant types. 



The irregular manner in which membrane-calcifications (equiva- 

 lent to membrane-bones, even if not osseous) are apparently deve- 



1 No bone-lacunae have hitherto been detected in this tissue. The present 

 writer has examined the mandibular splints cf Ischnacanthus and Acanthodopsis. 



