680 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
seen a beautiful series of minute rings and concentric circles, which I believe to 
owe their appearance to the original areolse ; other traces of the former existence of 
these areolse are preserved in the numerous brown semicrystalline points which are 
crowded together in the interior of some specimens. From the repeated conjunction 
of some of these fragments with the curious teeth of Diplodus gihbosus, I have been 
led to the conclusion that they have both belonged to one fish. On mentioning my 
idea to Sir Philip Egerton, I was glad to find that it accorded with the conclusion 
arrived at by our highest British authority in the field of fossil ichthyology. 
We are thus furnished with two very decided examples of bone in a fossil state, 
belonging to the chondriform type, which, as far as my investigations have been 
carried, appears to characterize the recent Plagiostomes. Many others will I have 
no doubt be eventually found to exist ; fragments, however small, when submitted to 
a careful microscopic examination, may thus be distinguished from the bones of the 
true osseous fishes. 
After having ascertained some of the leading features attending the structure and 
growth of chondriform bone when unaccompanied with that of any other type, I 
turned my attention to the same topics in connection with the bones of the osseous 
fishes. In this portion of the investigation, the inquirer is often tempted to draw 
general inferences from partial and limited data ; a proceeding, which I have learnt 
from experience to be especially dangerous in ichthyological studies ; I have so 
frequently been astonished to find that the results obtained were widely different 
from what I had anticipated, owing to the endless diversity of the modifications of 
the osseous structures of fishes. Still there are certain great groups, the individual 
members of which are constructed upon a common plan, and whose osteo-genesis is 
very similar. By submitting well-selected examples from each of these groups to a 
careful investigation, we may obtain a large amount of detailed information, and be 
enabled to deduce generalizations which will be found applicable to large numbers of 
the osseous fishes, without necessarily tracing the progress of every bone in each 
individual species. 
On examining a large number of Cycloid and Ctenoid fishes, we find that whilst 
the bones of some are so wholly osseous that scarcely a trace of cartilage remains in 
connection with them, others again preserve a large amount of cartilage in connec- 
tion with their skeletons to the latest period of their existence. The Pike may be 
taken as an example of the latter, and the Cod, Haddock, Perch, &c., as belonging to 
the former of these classes. 
Selecting the Pike [Esox lucius, Linn.) as a special subject of investigation, I have 
examined one by one, nearly every bone entering into the composition of its ske- 
leton ; making numerous sections of each, not merely in one fish, but in as many 
examples at different stages of their growth as I could conveniently obtain. From 
this series of observations I have been enabled to satisfy myself, in nearly every 
instance, as to the relations which the various osseous and cartilaginous portions have 
borne to each other. 
