FOSSIL MAMMALIA FROM THE STOiNESFIELD SLATE. 429 



The evidence is, therefore, strongly in favour of the view 

 that the common ancestor of marsupial and placental mammals 

 had teeth with many cusps of the Tritubercular sectorial pattern. 

 What, then, was the pattern of the molars of the ancestors of 

 both Monotremes and Ditremes? As yet we can give no 

 definite answer to this question ; but one thing seems extremely 

 probable, namely, that they were of an indefinite multituber- 

 culate pattern, which gave rise, on the one hand, to the elabo- 

 rate multituberculate teeth,^ and on the other to the Trituber- 

 cular sectorial. Thus the development of two longitudinal 

 rows of three cusps would give rise to the type of lower molar 

 common amongst the Multituberculata ; the fusion of the two 

 anterior of these cusps or the loss of one would yield the Tri- 

 tubercular sectorial tooth common amongst the Marsupials 

 and Placentals; while the loss of the inner cusps would result 

 in the formation of a Triconodont molar. The conclusion 

 reached is, therefore, that the primitive mammalian molar 

 bore a crown with several cusps. 



List of References. 



1. Agassiz. — Short note in the ' Neues Jahrbuch fiir Mineralogie und 



Geologic,' vol. iii, 1835. 



2. Blainville, Henki Duceotay de. — " Doutes sur le pr6tendu Didelphe 



fossile de Stonesfield, &c.," ' Comptes rendus Acad. Sci.,' 1838. 



3. Blainville, Henri Ddckotay de. — "Nouveaux doutes sur le pretendu 



Didelphe de Stonesfield," ' Comptes rendus Acad. Sci.,' 1838. 



4. Brodekip, W. J. — "Observations on the Jaw of a Fossil Mammiferous 



Animal found in the Stonesfield Slate," 'Zool. Journal,' vol. iii, 

 1827-8. Also in the 'Annales des Sci. Nat.,' vol. xiv, 1828. 



5. Bdckland, W. — "Notice on Megalosaurus," 'Trans. Geol. Soc.,' 2nd 



series, vol. i, 1824. 



6. Buckland, W. — ' Bridgewater Treatise/ 1836. 



Qa. Charlesworth, E. — * British Association Report,' 1854. 



1 It must not be forgotten that these teeth have only been found in forms 

 with a reduced dentition. 



