4 



SIR R. OWEN ON A TRIASSIC SAURIAN. 



skull selected for the profile view in fig. 1, PI. I. No' trace of 

 successional teeth, as in ordinary Saurians, has been discovered. 



Crocodiles and Alligators,, both existing and extinct, have two or 

 more teeth of canine proportions on each side of both upper and 

 lower jaws, the largest and most conspicuous being developed from 

 near the middle of the dental rank*; but these teeth differ in shape 

 from those of Mammalian Carnivores and Galesaurs, have thicker 

 crowns, subcircular in transverse section, chiefly differing in size 

 from the smaller teeth before and behind them. A similar character 

 and disposition of destructive canines is shown by the fossil jaws of 

 the Oolitic great extinct carnivorous Saurians — Megalosaurus, for 

 example. In the Triassic Labyrinthodonts the destructive and pre- 

 hensile laniaries occupy the foremost end of the dental series, are 

 four in number, two on each side, and by position in the lower 

 jaw would rank as " incisors " rather than canines f . 



In the poisonous Snakes the canines, in number and position, 

 resemble those in the upper jaw of carnivorous Mammals, but are 

 limited to that locality, and have the well-known modifications of 

 structure and attachment in purposive relation to the infliction of a 

 venomous wound J. 



As a rule the dental series, in existing Lizards, shows teeth of 

 nearly uniform shape, and either similar in size (see figs. 1, 2, 3, 4, 

 pi. lxvi., ' Odontography,' vol. ii.), or gradually increasing, chiefly 

 in breadth of crown, from before backwards (figs. 6, 7, loc. cit.). 

 Moreover the cement-clad roots contract bony union with the jaw- 

 bones, both upper and lower ; whilst in Galesaurus the teeth, besides 

 being distinguished, as in Mammals, by their differential character 

 as incisors, canines, and molars, are implanted freely in sockets of 

 due depth, the cold-blooded characters being manifested solely by the 

 greater number of teeth following the canines, and by the absence 

 of those developmental and formal distinctions which enable the 

 naturalist to classify them, in Mammals, as " premolars " and 

 " molars." 



Here, however, I may remark that some extinct Mammals of the 

 Oolitic period have retained the earlier Reptilian character of the, 

 geologically, older subject of the present communication. I refer to 

 the excess of number of the molar teeth in Amphitherium (Brit. 

 Toss. Mammals, 8vo, 1846, p. 29. fig. 15, p. 44. fig. 16), a character 

 still retained in the existing Australian Myrmecobius. Galesaurus 

 also resembles that and other Marsupials § in the number and 

 relative size of its upper incisors, while retaining the same number 

 below. 



One cannot rise from these comparisons without speculating on 

 the association of the degree oi mammalian dental resemblances 

 manifested by the teeth of the old Triassic Eeptile at the southern 



* ' Odontography.' vol. ii. (Atlas), pi. 75 a. figs. ] & 2. 

 t Tom. cit. pi. 63 a. figs. 4 & 5. 

 + Tom. cit. pi. 63. figs. 8, 9, 13. 



§ Tom. cit. pi. 98. fig. 1, Thylacinus ; fig. 2, Dasyurus ; fig. 3, Phascoyale ; 

 fig. 4 3 Myrmecobius. 



