226 LECTURE IX. 



plates of the dentinal pulp, and becomes co-calcified with theni, a 

 transverse section of such a tooth presents a series of interblended 

 wavy or labyrinthic tracts of thick dentine radiating from the centre, 

 and of thin cement converging towards the centre of the tooth. * An 

 analogous but more complicated structure obtains when the ra- 

 diating, wavy, vertical plates of dentine dichotomise, and give oiF 

 from their sides, throughout their course, numerous branch plates and 

 j)rocesses, which are traversed by medullary sinuses and canals with 

 their peripheral terminations dilated, and becoming the centres of lobes 

 or columns of hard dentine. The transverse section of such teeth 

 gives the appearance of branches of a tree, with leaf- stalks and leaves, 

 radiating from the central pulp-cavity to the circumference of the 

 tooth ; and I have called the fossil Fish in which this structure was 

 first detected, Dendrodus f . Thus, with reference to the main and 

 fundamental tissue of tooth, we find not fewer than six leading modi- 

 fications in Fishes: hard or true dentine (Sparoids, Lahroids, Lo- 

 phius, Balistes, Pycnodonts, Prionodon, Sphyrna, Megalichthys^ 

 Rhizodus, Diodon ; Scarus) ; osteo-dentine ( Cestracion, Acrodus, Le- 

 pidosiren, Ctenodus, Hyhodus, Percoids, ScicBnoids, Cottoids, Go- 

 bioids, and many others) ; vaso-dentine {Psammodus, ChimcEi-oids, 

 Pristis, 3fyliobates) ; plici-dentine (Lophiiis, Holoptychms^ Lepidostetis 

 oxyurus, at the base of the teeth) ; labyrintho-dentine {Lepidosteus 

 platyrhinus, Bothriolepis) ; and dendro-dentine (^Dendrodus) ; besides 

 the compound teeth of the Scarus and Diodon. 



One structural modification may prevail in some teeth, another in 

 other teeth of the same fish ; and two or more modifications may be 

 present in the same tooth, arising from changes in the process of 

 calcification and a persistency of portions or processes of the primitive 

 vascular pulp or matrix of the dentine. 



As might have been anticipated from the discovery of the varied 

 and predominating vascular organisation in the teeth of fishes, and 

 the passage from non-vascular dentine to vascular dentine in the 

 same tooth, the true law of the development of dentine " by centri- 

 petal metamorphosis and calcification of the cells of the pulp," was 

 first definitely enunciated and illustrated from observations made on 

 the development of the teeth of fishes. | 



* This remarkable structure attains its highest complication and forms the 

 largest proportion of the tooth in the gigantic extinct Batrachia, which I have 

 thence called Labyrinthodonts, and from which, therefore, I have taken the illus- 

 trations of that complex modification of dental structure in my " Odontography" 

 (pis. 63 6, 64, 04 a, 64 6). I had discovered in 1841 (lxxxvii.) the more simple 

 modification of this structure " at the base of the tooth in a few Fishes," but had 

 not then seen so complex an example in that class as Dr. Wyman (i.xxxvi. pi. v. 

 iig. 4.) and M. Assassiz (xxii. ' Sauroides,' 1843) subsequently described and 

 figured, in teeth of the genus Lepidosteus. j- cxxvi. pi. u. 



I In my Hunterian Lectures, delivered at the Royal College of Surgeons, May, 

 1839. See also Lxxxvirr ■- 784. ; and v. Introduction, and part i. passhn. 



