288 On the Skull of Tritylodon longevus, Owen. 
of this last statement I add here in Pl. X. fig. 3 a photograph 
of two newly prepared molar teeth of Diademodon brownt 
with ‘undivided roots *, and in Pl. X. fig. 5 a photograph of 
the newly prepared molar tooth of Cynognathus crateronotus, 
showing undivided root and a groove on the outer surface, 
indicating the beginning of a partial division of this root, as it 
seems, throughout its whole length. 
Il. Reptilian Characters. 
Divided nares ; 
Prefrontal bone T; 
Frontal bone not bounding the orbit. 
ras 
III. Characters both Mammalian and Reptilian. 
1. Septomaxillary bones; 
2. The terminal position of the anterior nares ; 
3. The backward position of the posterior nares ; 
4, Considerably diverging parietal bones ; 
5. Orbito- or alisphenoid (or orbitopalatine ?) ; 
6. No postorbital bar ; 
7. Brain-case antero-laterally closed. 
If we try to separate in this last group the characters that 
are primarily reptilian in T'retylodon trom those which are 
probably purely mammalian, we should regard only the 
existence of the septomaxillary bone as a pure reptilian 
character (common. both to Yritylodon and to Monotremes), 
So that Zrztylodon is to be regarded, in the first place, as a 
mammal, but as a mammal with some undoubtedly reptilian 
characters, the most primitive of the known Mammals. With 
its mixed characters, Tritylodon is a direct proof that the 
Mammals have their origin in Reptiles, most probably in 
theriodont Reptiles. 
* Seeley mentions (comp. “ On Diademodon” in Philos. Trans. 1895, 
pp. 1029-1041, figs. 5-10) three isolated molar teeth of Diademodon with 
undivided roots, but they seem to belong to lower molars. I add here 
in Pl. X. fig. 4 another photograph of two single-ruoted V-shaped teeth of 
Diademodon in section trom the specimen R. 8605 in the British Museum 
(mentioned by Mr. Watson, “ The Skull of Diademodon,” in Ann, & Mag. 
Nat. Hist. (8) viii. 1911, p. 814), one with open and the other with 
closed pulpa. 
“+ The existence of a prefrontal bone in mammals has been affirmed 
(comp. R. Cunningham, “ Note on the Presence of Supernumerary Bones 
occupying the Place of Prefrontals in the Skull of certain Mammals,” in 
Proc. Zool. See. Lond. 1889, pp, 76-77). 
