DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
699 
various fishes, are usually very different in their aspect from those with which the 
name of Havers has been connected by the anthropotomists. Similar complicated 
concentric systems of osseous lamellae, with their parallel arrangements of lacunae, 
have a definite existence in the scales of Megalichthys, and it is easy to trace a series 
of links which connect the latter with the more simple canals existing in the bones 
of the Pike ; a further transition from these to the similar passages seen in the scale 
of Dactylopterus (fig, \A a, h and c), and in the ganoin of Balistes (fig. 13 f, f), is 
equally easy. Much as the extreme forms differ, the whole appears to represent one 
homologous series. 
Another moot question, which derives some light from the preceding observations, 
is that of the nature of the opercular bones. Reference has been already made to 
the conflicting views of Professor Owen and M. Agassiz on this point ; the former 
gentleman regarding them as portions of the endoskeleton, whilst the latter con- 
siders them to be enlarged and modified scales belonging to the dermal exoskeleton. 
The distinctions between the structure and mode of growth of cycloid scales, and of 
opercular bones, are clear and obvious. The characters are wholly dissimilar ; whilst 
the genesis of the latter appears to be invariably connected with a primary deve- 
lopment of cartilage, we have not the least ground for supposing that this tissue is 
ever connected with the former of these structures. In the case of the Macropoma 
Mantelli from the chalk, the distinction is even still more obvious than usual, owing 
to the large development of beautiful lacunse, which has taken place in the true 
osseous opercula, and which correspond exactly with those seen in other portions of 
the endoskeleton, whilst nothing resembling them occurs in the interior of the true 
dermal scales. The existence of small points of kosmine on the surface of the oper- 
culum, resembling those covering the scale, does not invalidate this conclusion, since 
we find that closely analogous structures occur within the mouths of many osseous 
fish, connected with bones which are unquestionably portions of the endoskeleton. 
The time has not yet arrived when any attempt can be safely made to arrange 
the various processes of ossification existing in fishes, so as to enable us to deduce 
from them any general law. Why one kind of bone should exist in the Salmon and 
another in the Pike; why the rnembraniform osseous tissues of the Eel, the Lepidos- 
teus osseus and the Loricaria should abound in lacunse, whilst they are absent from 
the corresponding tissues of the Cod, the Haddock and the Perch ; and why the 
same tissues in the Lepidosteus and the fossil Saurocephalus should have in addition 
a large number of narrow tubes which also occur in the scales of Lepidosteus and Le- 
pidotus*, whilst these appendages are absent from the vast majority of ichthyal 
bones, are questions which we are not yet able to decide. The degree to which 
these several types prevail amongst the various families of fish is yet to be ascer- 
tained, and nothing but widely- extended observations will obtain for us the necessary 
information. Still it is very desirable that we should possess accurate knowledge on 
* Philosophical 'Fransactions, ut supra, tab. xl. figs. 2 b and 3 a. 
