SECONDARY MARSUPIALS, 275 
FAMILY SPALACOTHERIID. 
GENUS SPALACOTHERIUM. 
Spalacotherium, Owen, Mesozoic Mamm., p. 53 (1871) ; 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 S. ¢vicuspidens, 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/#. 
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 Z7iconodontida, or in a triangle 
like those of the Spalacotheridea. 
T32 
