40 
MR. C. S. TOMES ON THE STRUCTURE AND 
formed from the odontoblast layer of a simple pulp. It is abundantly permeated by 
tubes of larger calibre formed by the enclosure of, and containing, capillary blood- 
vessels. Example : Tooth of Hake. 
Hard unvascular dentine derived from the calcification of a pulp of simple form, 
passes through gradational forms in which the bone of the tooth is fluted as in 
Lepidosteus (fig. 12) into 
III. Plic [-dentine ; a tissue with true dentinal tubes, which is derived from the 
calcification of a pulp, the odontoblast- carrying surface of which has been rendered 
complicated by infoldings of its surface. Example : Tooth of Lahyrinthodon. 
And lastly, we have typical 
IV. Osteo-dentine ; a tissue devoid of true dentinal tubes (save in the form of a 
layer of hard dentine upon its surface) and derived from a calcification shooting through 
the whole substance of the formative pulp, so that it is not derived from a specialized 
odontoblast layer at all. The larger tubes do not contain capillaries, and its only 
complete distinction from bone lies in the fact of its development in a dentinal pulp, 
but not in the manner of that development. It is so closely akin to bone that the tooth 
of a Pike might be not inaptly described as a conical core of bone, furnished with a thin 
skin of hard dentine. 
In the foregoing definition of osteo-dentine it may be noticed that no mention is made 
of the two characters by which Professor Owen sought to distinguish that tissue, viz. 
the arrangement of the matrix in concentric rings around the vascular canals, and the 
presence of lacunae similar to those of bone. These characters have been intentionally 
omitted for several reasons ; the one, that there are many teeth the dentine of which 
I would, both from its development and its structure when perfected, class as osteo- 
dentine, in which neither of these characters is to be found, as, for example, the teeth 
of the Pike ; another, that a concentric arrangement of the matrix around the canals 
is to be met with in vaso-dentine sometimes, and that lacunae, or at all events spaces 
very similar in character, occur in dentine that no one would class as osteo-dentine ; 
whilst lacunae are absent both from bones and teeth in many fish whose teeth must, 
if my classification by developmental characters be adopted, be considered as consisting 
of osteo-dentine. 
But although these are adequate grounds for leaving them out of the definition, it 
may save misapprehension to add that a laminated arrangement of matrix, and spaces 
■ similar to bone lacunae, are, as might be expected, more commonly met with in the 
osteo-dentine than in any of the other forms of dentine. 
When I commenced the examination of the teeth of Gadidse, my object was not 
so much to investigate the nature of vaso-dentine, as to endeavour to ascertain how far 
minute structure was constant within the limits of a well-defined group. Unfortunately 
I have not as yet obtained materials sufficient to enable me to carry out my 
original purpose, to which I hope at a future time to recur, but I may here briefly 
indicate one or two of the facts which I have ascertained which bear on this question, 
