PURBECK FORMATIONS. 87 



in Plagiaulax Becklesii, fig. 8. Repeated researches have failed to bring to light any 

 other instance of a vertically replacing tooth ('dent de remplacement,' Cuv.), save that 

 ' premolar,' by development as well as by shape, which displaces the second or the first 

 and second of the primary or deciduous molar series in the Foephaga} 



This premolar {p 4, figs. 13, 14, p. 92), which assumes an antero-posteriorly extended 

 trenchant and vertically ridged character in Hypsiprymnidce, may be the homologue of jo 4 

 in Plagiaulax, but the proof from developmental character is Avanting. I deem it most 

 unlikely that any of the premolars in fig. 7, p. 86 should be destined to be displaced and 

 replaced by a vertical successor of larger size and similar character. There then remains 

 the possibility, assuming the trenchant, serrate, ridged teeth in Plagiaulax medius to be 

 permanent or non-deciduous primary ones, that they belonged to a female Plagiaulax 

 Becklesii, in which the incisor was not fully risen into place. The difference in the 

 shape of the coronoid process [c] may depend upon some marginal defect of that plate in 

 the small and delicate fossil." The condyle (c) may likewise have lost so much of its pro- 

 minence as would have brought it to the vertical parallel of the angle of the jaw {a), as in 

 fig. 8 and PL IV, fig. 10. 



If the parts as figured by Dr. Falconer were natural, they would indicate, with dif- 

 ference of size, a diff'erence of shape of jaw, which, as compared with that of the type 

 Plagiaulax Becklesii, should be interpreted as specific. They undoubtedly show the same 

 low position of the condyle, viz. depressed below the level of the molar alveoli — lower, 

 indeed, than in the type-specimen (fig. 8, p. 86). The figure also shows the wide emargina- 

 tion between the base of the coronoid (c) and the condyle {b), like that which is seen in 

 the mandible of Thglacinus (fig. 5, p. 74), and to which the term ' neck' or ' peduncle' of 

 the condyle is sometimes applied. 



From the seemingly more distinct and prominent angle (fig. 1 , a) a. ridge is con- 

 tinued forward and upward, bounding below the external crotaphyte hollow, at a higher 

 level than the inverted angle and lower margin of the jaw bounds the internal or ptery- 

 goid hollow in Plagiaulax Becklesii (PI. IV, fig. 10).^ 



In the well-preserved ' ascending ramus' of the jaw of Plagiaulax Becklesii (PL IV, 

 fig. 10, 10 a) there is no trace of a solution of continuity effecting a communication between 

 the external and internal crotaphyte depressions near the entry of the dental canal, as 



' Owen's "Fossil Marsupialia from the Caves of Wellington Valley," May 8, 1838, in Mitchell's ' Three 

 Expeditions into the Interior of Eastern Australia,' 8vo, vol. ii, p. 360, pi. 30. 'Classification of the Mar- 

 supialia,' ' Proceedings of the Geological Society of London,' Jan. 8th, 1839. 



2 Dr. Falconer, who describes this fossil, in a P.S. to his original paper, as "a fifth specimen of 

 Plagiaulax'^ received since that paper was written, states, " The true molars, if present, are concealed by 

 the flap formed by the anterior margin of the coronoid process. This part of the jaw has been slightly 

 crushed."— Loc. cit. (1857), p. 271, op. cit. (1808), p. 421. 



3 " The base of the coronoid is occupied by a deep depression bounded on the lower side by a raised 

 ridge, which sweeps round from the inferior part of the condyle, to be continued into the anterior margin of 

 the coronoid process." — Falconer, loc. cit., 1857, p. 271, tom. cit., 1868, p. 421. 



