258 
()\ THK STllUCTURE AND CLASSIUCATION 
tlie ancestors of the rich fauna of placentals found in the Puerco, if not in the kno^vn 
Jurassic and as yet unknown Cretaceous mammals] The probable features of the 
dentition of the stem type have already been outlined from a comparison of the 
Mesozoic genera, on page 247. The jaw had an unankylosed symphysis, a mylo- 
hyoid groove and a distinct coronoid, angle and condyle. 
^^'e may now consider the limited evidence we have which bears upon the 
zoological relations of these mammals. The Protodonta are considered as a distinct 
order and are not included in this discussion because nothing is known of their con- 
temporary or succeeding fauna. 
Rel.\tions to the Marsupialia. 
The only distinctive features of the modern marsupial mandible and dentition 
arc the inflection of the angle and the peculiar reduction and succession of the teeth. 
But we find the angle is not inflected in Tardpes nor in some species of Pera- 
thermm,' showing that this is not an essential marsupial character. The condyle and 
angle var\' in jwsition and relation directly according to the function of the jaw. 
'I'hey arc low and confluent in the carnivorous forms, lofty and separate in the 
insectivorous forms. A mylohyoid groove is occasionally developed ; it was described 
by Owen in Mi/rmecoldus,^ which is also multidentate, the teeth numbering 54, (i 1, 
c 1, pm I, m I). There are in most genera four molars and never more than three 
premolars. Oldfield 'fhomas has recently confirmed Flower’s hypothesis that the 
Marsupials have lost one premolar, enabling us to homologize this with the placental 
•series. The canine is bifanged in Chceropiis and occasionally in Perameles. Four 
lower incisors are sometimes developed, e. g., Didelphys, although the typical number 
is three. 
I'nmisUikable marsupial characteristics are found among the Tricatiodontuke 
and Aniphitherilda-. The mylohyoid groove is always present. The angle unfortu- 
nately is rarely preserved ; so far as known, it is not inflected in the latter family. It 
is distinctly inflecteil and shelf-like in Phascolotherium. and Spalacotherium, and fully 
marsupial in Tricotiodon. The primitive number of incisors is four, but, as shown 
upon i>age 248 is reduced to three in the carnivorous series, by the loss of i 4. The 
canines arc bifanged in the greater number of genera. The premolars are almost 
coi„tantly four in number, and their mode of reduction and succession is strikingly 
mnniupinl. As slioun U|>on page 247 the Amphitheriidm lose the second premolar, 
rvluch .s by no means a common mode of reduction, yet it corresijonds with what has 
• ThI. oWrvMion ,pp„„ to h.ve boon a mi.toie, s,e Append!,. 
