192 



H. SPENCER HARRISON. 



tooth/ with a coustant series of successors, the outer one in 

 h and c being really a maxillary tooth. The dotted tooth of 

 the fifth dentition has not been found in any of my specimens, 

 but I believe it will be found to occur in slightly older 

 individuals.^ 



We are now called upon to account for the presence of 

 the posterior uniform series, and for the lack of synchronism 

 in its first appeai-ance in the different dentigerous bones. 



That the uniform series must be regarded as a later ac- 

 quirement than the other teeth, I consider has been suf- 

 ficiently proved by the facts adduced as to the occurrence of 

 a cessation of activity when the alternating series have 



Pmx. 



Mx 



Diagrams to illustrate the dentitions represented in tiie premaxiilary teetli up 

 to the stage of my largest specimen (compare witli figs. 20, 21, and 22). 

 Dentitions: — I, black; II, iiorizontal lines; III, perpendicular lines; 

 IV, dotted ; V, blank, Mx, maxilla. Pmx, premaxilla. M, middle line 

 of jaw. a. Purely theoretical, showing all the successional teeth at one 

 time. b. Modifled from figs. 20, 21, and 22 of Stage T, and a little 

 later, c. As seen in my largest specimen, with hypothetical tooth added 

 (dotted outline). 



been completed. What was the cause of the development of 

 this new set of teeth can only be conjectured. It may have 

 been due to a general increase in size of the animal, or to the 



' In this connection it is interesting to note that in Zittel's 'Palaeontology ' 

 (23) Homceosaurus of tiie Jurassic, which is placed next to Hatteria in 

 tlie family Sphenodontidse, is stated to have the "Zwischenkiefer paarig 

 am Vorderrand mit einem einfachen sclineidenden Zahn besetzt." 



' See Addendum, 



