﻿SECONDARY MARSUPIALS. 275 



FAMILY SPAT.ACOTHERTID.E. 



GENUS SPALACOTHERIUM, 



Spalacotherium, Owen, Mesozoic Mamm., p. 53 (187 1) ; Lydek- 

 ker, Cat. Foss. Mamm. Brit. Mus., pt. v., p. 292 (1887). 



In this family and genus the cusps or cones of the molar 

 teeth (as shown in Fig. 4 of the diagram on page 274) are 

 arranged in a triangle, of which the apex points inwards in the 

 upper, and outwards in the lower jaw. In the lower jaw there 

 are only three such cusps to each tooth, but in the upper teeth 

 there is an additional posterior cusp. In structure these teeth 

 are similar to those of the Marsupial Mole. All the members 

 of the family are exceedingly minute, the lower jaw of the 

 typical ^. tricuspidens^ of the Dorsetshire Purbeck beds, scarcely 

 exceeding an inch in length. The number of teeth in the 

 lower jaw was large, there being three pairs of incisors, one of 

 canines, four of premolars, and six of molars. Typically 

 occurring in the Dorsetshire Purbeck rocks, the family is 

 represented by closely allied forms in the Upper Jurassic rocks 

 of the United States. 



FAMILY AMPHITHERIID.F:. 



This extensive family is taken to include a number of forms, 

 of which it is not easy to give a collective definition, although 

 they appear to differ from the two preceding families in having 

 four pairs of incisor teeth, like the Opossums. The lower 

 molars never consist solely of three simple cusps arranged in a 

 straight line like those of the Triconodontidce^ or in a triangle 

 like those of the Spa/acothertidce. 



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