696 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
fishes consist of similar membraniform bone, here exhibit the same areolar struc- 
ture as the plates of the vertebral centrum, whilst they are surrounded by a peri- 
osteum which preserves all the characteristics of a genuine fibro-cartilage. The same 
remark applies to the osseous laminae which encroach upon the receding cartilage 
along the ossifying extremities of the bone, which laminae are of course also formed 
in the fibro-cartilage. These circumstances leave little doubt upon my own mind, that 
in the case of the Salmon, the structure about to be calcified, whether lining a can- 
cellus or existing as a periosteum, is merely a fibrillated modification of the primary 
cartilaginous matrix, and it becomes increasingly probable that the same is the case 
with the homologous portions of other osseous fishes. The subject is too important 
in its bearings upon some branches of physiological science to allow of rash or pre- 
mature conclusions, and consequently I would throw out the above suggestions in a 
spirit of due caution, being well aware of the vast amount of detailed observations 
yet requisite before the general principle can be established. 
This structure of the bones of the Salmon also teaches us, that though the differ- 
ences between membraniform and chondriform bone are usually clear and well- 
marked, yet the extreme forms of each may be connected by inosculating links, 
which break down the artificial distinctions that our imperfect philosophy would set 
up. We have learnt the same lesson from the snout of the Saw-fish. 
It will scarcely be necessary to prolong this memoir by recapitulating the various 
conclusions which the observations detailed in the preceding pages have enabled 
me to deduce, since they have been recorded as we proceeded with the details 
of the several topics. There are however two or three questions upon which the 
preceding inquiries have some bearing, and which require a passing notice. 
We have already found that the non-cellular, fibrous membranes of Cycloid and 
Ctenoid scales, the fibro-cartilages of the Salmon and Saw-fish, and the true cartilages 
of various other fishes, appear to be calcified in one common way, viz. by the formation 
of small calcareous granules, which are not interposed between, but actually incorpo- 
rated with, the several tissues in which they are developed. In many of the examples 
which I have recorded, these granules are so large as to be easily examined, whilst 
the degrees to which their size is capable of being increased by the external addition 
of calcareous atoms, whieh are not themselves discernible, vary in different tissues 
and species of organisms. Analogical reasoning renders it probable that the calcifi- 
cation of such tissues as human teeth and bones may have been accomplished in the 
same way ; only through the ageney of granules which rarely attain a sufficient size 
to become visible even under high magnifying powers. 
Another still more important inquiry suggested by the growth of the various forms 
of kosmine, has reference to the relation which they sustain to true dentinal structures. 
We have seen that a great variety of kosmine tissues exist, which exhibit the closest 
resemblance to several of the modifications of dentine described by Professor Owen. 
Thus in the areolae of the MegaUchthys, the dermal teeth of the rostrum of the 
