﻿50 Sir PhilijJ Grey-Egerton — On some New Pycnodonts. 



row characteristic of the larger teeth, but it occurs in the centre of 

 the denticle, and is deeper in proportion to the size of the tooth, so 

 that it is hardly altogether effaced by use. The four posterior teeth 

 of the third row (PL III. Fig. 1 c) are smaller than those of the second, 

 and the outline is less elliptic and more irregular. These also have 

 the central furrow described before, but modified in harmony with 

 the form of the teeth. The teeth in front of these (PL III. Fig. 1 d) 

 are in a double row, and seem to have arisen from divided germs, 

 each pair making up the form and size of one of the posterior teeth, 

 suggesting the idea that the single teeth of the hinder portion of the 

 rank are replaced by twin teeth in the front of the mandible. This 

 peculiarity will be alluded to more particularly hereafter. The 

 result is a great irregularity in this part of the dentition. Some of 

 these small teeth are elliptical, some more or less rounded, some 

 trigonal ; some have the larger axis to the front, others coincide 

 with the two inner ranks in having the larger axis transversal. 

 They all, however, have a central pit varying in form with the out- 

 line of the tooth. The mandibular bone (PL III. Fig. 1 m) extends 

 an inch or more backwards beyond the dentigerous area ; it is close- 

 grained and solid, and measures an inch in thickness in the middle, 

 and thins off a little towards the symphysis, and more so to form the 

 outer edge of the mouth. 



Before attempting to determine to which genus of the Pycnodont 

 family this specimen is referable, it will be necessary to notice 

 some of the discrepancies which occur in the writings of those authors 

 who have treated of this subject with regard to the number of the 

 ranks of mandibular teeth. Agassiz ' says, " The lower jaw is car- 

 petted with large teeth arranged in three or five ranks on each side." 

 Pictet" follows this dictum. Wagner and Thiolliere,^ on the contrary, 

 maintain that the correct number is four ; but in the figure given by 

 the latter of Pycnodus Bernardi (pi. 5, fig. 2), five rows are distinctly 

 shown. Sauvage* divides the genus into those with four rows and 

 those with more. The great opportunities afforded me of examining 

 these interesting remains of a family entirely extinct since the 

 Miocene age have led me to think that much irregularity obtains in 

 the development of the marginal rows of tritoral teeth, and that 

 these variations are not of specific value except in cases where they 

 are constant. In Pycnodus BucTdandi, Hugii and parvus there are 

 always five and sometimes six rows. The inner row when present 

 is invariably composed of small teeth, which are sometimes irregular, 

 sometimes have fallen away through use or age, and in one specimen 

 in my collection are duplicated. The second row is always com- 

 posed of the largest teeth and is generally constant in character. 

 The third row is also for the most part regular, but composed of 

 much smaller teeth than the second. The outer rows are subject to 

 great irregularities in number, form, and position. In some cases 



' Poissons Fossiles, voL 2, pt. 2, page 183. 



2 Traite de Paleontologie, vol. 2, p. 197. 



3 Poissons Fossiles du Bugey, p. 11. 



* Poissons des Formations Secondaires du Boulonnais. 



