347 



My meaning may perhaps be made more clear by a comparison ; 

 the incisors of Galeopithecus, as is familiar to every student of 

 odontology, have their edges divided into comb -like processes, each 

 of which on transverse section presents an axial pulp canal from which 

 radiate dentinal tubes. 



If we imagine a number of these processes packed side by side, 

 and without their enamel coat, we should have a structure practically 

 exactly like a transverse section of a tooth of Myliobates. 



But these processes of the tooth of Galeopithecus are according 

 to any conceivable definition made of true dentine, and so a term such 

 as trabecular dentine applied to the tooth of Myliobates, which might 

 also be described as a series of parallel denticles of true dentine, 

 does not seem to help the understanding of its nature. 



On the other hand there are here, as in all other kinds of dentine, 

 transitional forms which make classification difficult, and it was in 

 view of these that I wrote: "Dentines which might be regarded as 

 aggregations of fused parallel denticles, as forms of folded dentine, or 

 as a transition towards a very regular osteodentin e. Examples Mylio- 

 bates, Pristis" (Dental Anatomy, p. 81). 



Dr. Rose has discarded my Subdivision "Plicidentine" which I 

 had defined as a tissue "permeated with dentinal tubes, which radiate 

 from a pulp chamber rendered complex in form by foldings in of its 

 walls. Examples Lepidosteus, Labyrinthodon . . ." and most of the 

 dentines therein included seem to have found their way into his 

 trabecular -dentine class. Here again I cannot regard the alteration 

 as altogether happy, as plicidentine appears to me to be a useful 

 term, and quite descriptive of certain modifications of dentine. 



If we take such tooth as that of Varanus, all its upper portion 

 is a simple hollow cone and consists of what must by every one be 

 admitted to be ordinary true dentine, but towards the base of the 

 tooth it gradually becomes folded in, so that the surface is fluted. 



Now every possible gradation exists between th's and the more 

 complex foldings met with in Lepidosteus, Labyrinthodon and Dendrodus, 

 which last Rose classes as trabecular dentine. 



INforeover he figures a cross section of a fossil tooth which so far 

 as the arrangement of the dentine goes would almost pass for a cross 

 section of the base of a young tooth of Lepidosteus and tells us, ex 

 hypothesi, that this contains vitro-trabecular dentine which was not 

 developed under an epithelial sheath. 



But the development of the tooth of Lepidosteus is perfectly well 

 known and I have figured (Dental Anatomy, Fifth Edition, p. 63) it. 

 There the dentine is forming from an odontoblast layer in quite the 



