110 
No. 2. — DESCRIPTION OF THE DECIDUOUS PRE MOLAR 
OF NO TO THE JR.IUM MITCIIELLT OWEN. 
In marsupials the dental series contains but one replacing' tooth 
— the so-called “permanent premolar.” In many of the Macro- 
podidae it replaces two teeth, the “milk premolar” and the “milk 
molar,” but as a rule it has but one predecessor, the “milk pre- 
molar.” The milk premolar of Thylacynus is developed and ab- 
sorbed before the animal is out of the foetal stage, and, in the case of 
PhascolomySj the deciduous tooth is merely vestigeal. 
Our knowledge of the dentition of extinct forms is very incom- 
plete; in regard to the D-iprotodontidae the remarks of Lydekker in 
188b,* “So far as we know at present there is no evidence of any 
tooth change or of the presence of a deciduous pm. 8 in either 
I)i protodon or Xtffllherium” do not appear to have been questioned 
or modified by more recent discoveries. 
A specimen from the Mammoth Cave, consisting of a small 
fragment of the right side of the skull of a young individual and 
including the facial portion from the orbit to the socket of the in- 
cisor with the dentition and the anterior portion of the palate is, 
therefore, of particular interest, because of the light it throws upon 
the dentition. 
The teeth present consist of the deciduous premolar in position 
but showing no trace of wear, and the posterior molariform tooth, 
still in its formative cavity, but evidently ready to emerge and take 
its place in the tooth line. 
On account of the swoollen state of the maxilla below the infra- 
orbital foramen, an opening' was made in the wall of the socket of 
the incisor disclosing the presence of an imperfectly formed suc- 
cessor to the milk -premolar. There is therefore no doubt that 
Nololheriu m, like the majority of the marsupials, possessed a de- 
ciduous premolar and a replacing tooth. 
The deciduous tooth is triangular, with a prominent crest ex- 
ternally and a well marked tubercle on a distinct talon at the postero- 
internal angle of the crown; this tubercle is connected with the cusp 
by an almost obsolete bridge across the floor of the intervening 
valley, which is closed internally and posteriorly by a strong sinuous 
cingulum. This ridge ascends the outer cusp anteriorly, gradually 
merging into the crest, but posteriorly it rises up the hind edge of 
the cusp forming a distinct prominence in line with the cusp but 
separated from it by a well marked notch. The highest part of the 
* Annuls and Magazine of -Nat. Hist. (6), Vol. III., No. 14; Feb. 1889, p. 151. 
