XIV 
INTRODUCTION. 
fish which all will agree is a typical Elasmobranch. The “ production 
of the pectoral arch into long backwardly directed processes in Dijpla- 
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. Einally, the contention that the Acanthodii may be a degene¬ 
rate branch of the “ ganoids ” that has followed and even descended 
beneath the Chondrostean Polyodontidse, 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-1 ayer 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 , 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-lacunas have hitherto been detected in this tissue. The present 
writer has examined the mandibular splints of Ischnacanthus and Acanthodopsis, 
