38 
MR. C. S. TOMES ON THE STRUCTURE ANU 
contrasting with that of the Pike, which is formed from a simple pulp by an irregular 
calcification running through its substance, and which is only in very small part 
formed from odontoblast cells. 
Dentine which has been formed from a convoluted pulp may attain to great com- 
plexity of structure, as happens in the Labyrinthodon, or the sub-division of the 
formative pulp may result in the production of a tooth which might be regarded 
as an aggregation of denticles, as happens in the Myliobates „ 
But although in this way considerable variety is arrived at, and the tissues bear a 
more or less close resemblance to osteo-dentine, yet there is one character which will 
generally serve to distinguish these from it. 
Each of the larger canals in the completed tissue forms an axis from which dentinal 
tubes radiate with some degree of regularity, these dentinal tubes having resulted from 
the calcification of the layer of odontoblasts with which each subdivison of the formative 
pulp was clothed ; whereas, in osteo-dentine, like that of the Pike, no such distinct and 
regular systems of dentinal tubes radiate from the larger canals. 
The true character of dentine derived from a convoluted pulp is not badly expressed 
by the term Plici-dentine, already applied by Professor Owen to some examples of 
this structure, and it would embrace dentine of every degree of complexity, from that 
of such teeth as those of the Lepidosteus, which are simple in their apices and not very 
greatly complicated at their bases, to such as those of the Labyrinthodon. 
An exceedingly instructive modification of dentine structure is to be found in the 
teeth of the Sparidse, of which I have examined Sargus ovis, Chr-ysophrys (?), and 
Pagellus. 
The manner in which their teeth are supported is very curious ; if a longitudinal 
section of a front tooth and the bone beneath it of Sargus or of Chrysoplirys be very 
carefully made (the tooth is easily broken off at the level of the bone, and the section 
can only be made by a skilled hand with a lapidary’s wheel), it will appear to the naked 
eye as though the tooth were furnished with a long root, half as long again as its 
crown, implanted in a socket of bone to which it is anchylosed ; whilst at its upper 
part this “ root,” if such it can be called, is sharply defined, at its lower end or apex- 
it merges into the surrounding and subjacent bone. 
It has already been incidentally mentioned that the tooth readily breaks off at the 
level of the surface of bone, easily parting from its implanted portion ; if a section 
such as that described be rubbed down and examined microscopically, it will be found 
that there is a rather abrupt change of structure at the point alluded to. The out- 
standing portion or crown of the tooth (d in fig. 23 and 24, Plate 3) is composed of 
hard fine-tubed dentine coated with enamel ; the implanted portion (d' in same 
figures) of vascular dentine. The pulp cavities of the two segments of the tooth are 
continuous and of the same diameter, and so are the walls of dentine (in fig. 17, the 
pulp cavity of the implanted portion is not shown, because the section is oblique, and 
does not pass truly along the long axis of the tooth). In tracing the development 
