XX 
INTRODUCTION. 
although there are no paired appendages in the former genus, we 
are inclined to think that another noteworthy point of resemblance 
occurs in the appendicular skeleton, the rows of plates in the 
paddles of the Asterolepidae being an extreme modification of the 
arrangement observed in the azygous fin-membranes of the Cepha- 
laspidge, and markedly different from the actinotrichian development 
by which the fin-rays of ordinary fishes arise. Even the support of 
the anterior border of the dorsal fin of Pterichthys is not a true spine, 
but merely a longitudinally bent (perhaps primitively double) scale. 
DIPNOI. 
Concerning the evolution of the Dipnoi, palaeontology as yet 
affords no information. So long ago as the Devonian period, there 
were members of the subclass agreeing precisely with the existing 
Ceratoclus in the development of the fins and the axial skeleton of 
the trunk. At that remote period, too, the chief part of the 
dentition had assumed the form of great plates upon the splenial 
bones and the palate ; and the principal difference between such a 
type as Phcineropleuron and the existing genus just mentioned 
seems to consist in the comparative fewness of the cranial roof- 
bones in the latter and the absence of membrane-bones on the 
margin of the jaw. The typical Dipnoi of the Devonian period had, 
indeed, already become more specialized than any known in later 
times; Dipterus exhibiting differentiated dorsal fins and a hetero- 
cercal tail. 
The latter fact is of all the more interest when the tendency of 
modern research in regard to the Coccosteus-like fishes is taken 
into consideration. According to existing diagnoses, these fishes 
must be assigned either to the Dipnoi or to the Teleostomi; and 
the extremely specialized character of their iiaired fins, so far as 
known, proves that, wherever they be placed, they occupy a com¬ 
paratively high position. If they are Teleostomi, they pertain to 
the Actinopterygian order, and hence ought to exhibit a well- 
developed hyomandibular bone. At least, in every undoubted 
Actinopterygian Teleostome possessing ossifications equal in extent 
to those of Coccosteus and its allies, the hyomandibular bone is both 
large and considerably ossified. In the extinct group now under 
discussion, how r ever, such a bone is not exhibited even by the most 
exquisitely preserved specimens. On the other hand, all appear¬ 
ances in the crania of Dinichthys and allied genera from the Waverly 
