THE rURBECK BEDS AT SWANAGE, DORSET. 



379 



molars is rather smaller than the others ; and, as in the type speci- 

 men alluded to above (plate iii. figs. 7, 7 a, loc. dt.\ the base of 

 the coronoid process hides part of the last tooth. Prof. Owen con- 

 siders that this character may indicate that the jaw described by 

 him belonged to an individual not quite fully grown. Assuming 

 such to be the case, the fourth molar in the present specimen would 

 point to a mature animal, though, for a similar reason, to one not 

 yet adult. In point of size, the last premolar and the three anterior 

 molars agree as nearly as possible with the corresponding teeth in 

 the type specimen of Triconodon mordax. 



The question, therefore, to be settled is, Does the new specimen 

 belong to this last-mentioned genus and species or not ? 



Prof. Plower, who has kindly examined it, and compared it with 

 those found by Mr. Beckles, which are now in the British Museum, 

 thinks, on the whole, that it may probably be referred to this species, 

 and ingeniously suggests two hypotheses to account for the extra 

 tooth of true molar form. They are as follows : — 



As is well known, the deciduous teeth of the Marsupials, to 

 which group the Mesozoic mammals have hitherto been assigned, 

 consist of a single tooth on either side, the tooth which replaces 

 this deciduous one being the last premolar. Assuming, then, that 

 this tooth is still in situ, and is represented by the third tooth, 

 counting from the symphysis, the dental formula will be p. m. 4 

 m. 3, which agrees with the type specimen of Triconodon mordax 

 in the British Museum ; and the first hypothesis is that the jaw 

 belonged to a younger individual than any previously found, with 

 the single milk-tooth in position. 



The second hypothesis is to assume that all the teeth preserved 

 belonged to the permanent set, when the dental formula will be 

 p. m. 3, m. 4. In this case the four molars agree in number with 

 those found in the adults of all recent Marsupials, and indicate a 

 more fully matured specimen than any hitherto discovered. 



A third hypothesis is to assume that it belongs to an altogether 

 distinct species ; but this, considering the close resemblance to 

 Triconodon mordax, appears hardly necessary. 



The lower border of the ramus is, in its present condition, nearly 

 straight ; but it has been much crushed, and w T hen recent was, with- 

 out doubt, slightly curved. 



A small outlet of the dental canal opens under the foremost root 

 of the third premolar (the first which is preserved) ; and there are 

 traces of one, if not of two, other outlets anterior to this. 



The total length of the jaw is 1 J inch. 



Disctjssion. 



Mr. K. Willett said that he hoped the specimen would be accepted 

 by the authorities of the School of Mines, and placed in the collec- 

 tion there. 



Prof. Dttncat* spoke of the importance of the communication and 

 the thoroughness which it showed. The jaw was a highly developed 



