ORDER MARSUPIALIA. 1 273 



ated. The cheek-teeth differ from those of all other Marsupials in 

 having the fangs imperfectly divided, and thus approximate to those 

 of certain Anomodont Reptiles. There are seven true molars, the in- 

 cisors are spaced, there is long diastema behind the canine, and the 

 three premolars are of a very simple structure. The crowns of the 

 true molars consist of one main cusp, with small accessory cusps ; 

 and it would appear that the teeth of the upper and lower jaws 

 mutually interlocked. This type of tooth is regarded by Professor 

 Osborn as the most archaic yet known. Microconodon, which 

 occurs in the same deposits as Dromatherium, is an allied but 

 smaller form. 



Family Amphitheriid^e. — This Mesozoic family is provisionally 

 taken to include a number of small and imperfectly known forms, 

 some of which are regarded by almost all writers as undoubtedly 

 Marsupials, although others have been referred by Professor Osborn, 

 with some hesitation, to the Insectivora, under the name of Insec- 

 tivora Primitiva ; those which are retained by that writer in the 

 present order being termed Prodidelphia. Although it is quite 

 probable that with fuller information the two sections into which 

 this family is divided may be raised to the rank of separate families, 

 yet the evidence brought forward by the writer mentioned above in 

 favour of referring some of the genera to the Insectivora appears to 

 be insufficient ; no members of that order having more than three 

 lower incisors or more than the normal three molars of the other 

 Placental Mammals. 1 Considerable confusion has arisen in regard 

 to the dentition of many of the members included in this family 

 owing to the circumstance that in the mandible only one side of 

 the teeth is generally seen, so that several genera have been made 

 upon the evidence of remains of a few closely allied species. The 

 clearing up of this confusion is mainly due to the careful observa- 

 tions of Professor Osborn. In this family all the genera are char- 

 acterised by a channel on the inner side of the mandibular rami 

 known as the Mylohyoid groove (fig. 11 47) — a feature occurring in 

 many recent Polyprotodonts. The number of lower incisors was 

 probably always four (as in the modern Didelphyidce), and the 

 lower true molars are frequently in excess of that number, as in 

 Myrmecobius alone among existing heterodont Mammals. These 

 true molars may either consist of three or more cusps arranged in a 

 single line, or they may be differentiated into a tritubercular blade 

 followed by a posterior heel or talon. The premolars are very 

 generally four in number, but they may be reduced to three, or 

 perhaps two. Not unfrequently, as in the existing Peramelidce, the 

 root of the canine may be grooved. 



1 Judging from his latest memoir on the subject, it is probable that Professor 

 Osborn would now considerably modify his views as to these divisions. 



