668 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
parallel with the pulp(?)-cavity. With one or two unimportant exceptions, the de- 
scription of the structure of the fossil Coelorhynchus applies, even in its most minute 
details, to that of the recent spine. In the latter example, I have not seen the long 
semilunar canals represented by figs. 36g and 37 d^. There is also some difference 
in the construction of the bases of the two examples. Coelorhynchus has always been 
regarded by iclithyologists as representing the true premaxillary bones of the fish to 
which the fossil originally belonged. This idea received some confirmation from the 
circumstance, that whilst at its apex it possessed a single central cavity, at its base 
this canal was not only divided into two by an intervening calcareous septum, but 
eveti the septum itself was duplex, consisting of two plates which were readily sepa- 
rated. Hence the opinion was adopted that the organism consisted of two separate 
bones, closely united, especially at the apices. I had always some difficulty in recon- 
ciling my observations with the above interpretation of the homology of this struc- 
ture, especially since I could not detect a very clearly defined line of demarcation be- 
tu^een the two supposed bones near their upper extremities. At length, however, I 
thought I had detected it, and figured it accordingly. Since then I have obtained 
more perfect sections of the fossil, through the kindness of Sir Philip Egerton, and 
am now convinced that no such line does exist. The structure is one entire organism, 
only having a bifid base and a corresponding bifurcation of the central canal. I 
have hitherto seen no dermal spine exhibiting a kosmine structure, made up of two 
lateral elements, as is sometimes the case with the true osseous dorsal ravs. Coelo- 
rhynchus is, I am convinced, no exception to this general law, and from the light 
now thrown upon it by the study of the recent examples just described, I have no 
hesitation in arriving at the conclusion that it is a fossil example of a dermal spine of 
a large Ostraciont fish. The future discovery of the scales is a doubtful matter. It 
is possible, that like those of the specimen from which my spines were taken, they 
may have been principally membranous ; still it is highly desirable that they should be 
sought for at the localities where the Coelorhynchus is found. The new aspect which 
this fossil now assumes may render it desirable to change its generic name, since the 
present one conveys a false idea as to its true anatomical homology. 
Having completed the examination of as many modifications of scale-structures as 
fell within my reach, it became a question of considerable interest to ascertain what 
physiological relation could be traced between their structure and mode of develop- 
ment and that of the bones constituting the endoskeletons of this group of vertebrate 
animals. The overthrow of many of the old notions respecting the genesis of mam- 
malian bones by Professor Sharpey, has rendered it desirable that all forms of ossific 
growth should be subjected to a new and rigorous examination, in order to ascertain 
how far these collateral witnesses give support or otherwise to the highly philosophic 
views that he has enunciated. 
I have employed the same method of investigation in this portion of the inquiry 
* Ut supra. 
