56 Mr. Owen on the Thylacotherium. 



of the tooth than is the enamel or its analogue in the teeth of any species of 

 Reptile, recent or fossil. The six molars, anterior to the one in place, are broken 

 off close to the sockets ; both the fifth and fourth false molars are entire : the an- 

 terior cusp has the same superior size, as in the first specimen. The thick enamel 

 coating, and the silky iridescent lustre of the compact ivory, are beautifully shown 

 in these teeth. 



The third and second grinders are more fractured than in the first specimen, but 

 sufficient remains to show that they possess the same form and relative size ; but 

 the most interesting evidence, as regards the teeth, which the present jaw affords, 

 is the existence of the sockets of not less than seven teeth, anterior to those above 

 described. Of these sockets the four anterior ones are small and simple, like 

 those of the Mole, being more equal in their size and interspaces than in the Di~ 

 delphys. The fifth socket contained a small premolar with double fangs, and so 

 likewise the sixth and seventh. Thus the two false molars, with perfect crowns in 

 the present specimen, are the eighth and ninth teeth, counting backwards, or the 

 fourth and fifth of their class, viz. premolares, or false molars. 



Thus, as regards the genus Thylacotherium, we have now evidence that its 

 dental formula must include thirty-two molars in the lower jaw, — sixteen on 

 each side : that these, instead of presenting an uniform compressed tricuspid 

 structure, and being all of one kind, must be divided into three series, as regards 

 their form : five, if not six, of the posterior teeth are quinque-cuspidate, and must 

 be regarded as molares veri. Some of the molares spurii are tricuspid and some 

 bicuspid, as in the Opossums ; but they are six, if not seven, in number : anterior 

 to these are four simple teeth, of which the fourth may be regarded as the repre- 

 sentative of the canine, and the anterior three as incisors. Thus the Thylacothe- 

 rium differs considerably from the genus Didelphys in the number of its teeth : in- 

 deed, at the time when Cuvier wrote, no mammiferous ferine quadruped was known 

 to possess more molar teeth than the Chrysochlore, which has nine molars on each 

 side of the upper jaw, and eight molars on each side of the lower jaw. 



The Chrysochlore, however, is not the only Mammal in which the molars exceed 

 the number usually found in the unguiculate Mammalia. 



In the Transactions of the Zoological Society for the year 1836 (vol. ii. p. 149), 

 Mr, Waterhouse published a description of an Australian Marsupial, forming the 

 type of a new genus (Myrmecobius) , and having nine molars on each side of the 

 lower jaw, besides one small canine and three simple conical incisors*. 



* The six posterior grinders of Myrmecobius are compound and multicuspidate ; the three anterior 

 ones are molares spurii and tricuspid ; with the exception of the last, they gradually diminish in size 

 as they advance forwards (PI. V. fig. 2). 



