102 OSTEOLOGICAL GALLERY. 



in their dentition the Dasyuridce, and in the structure of their 

 feet the Phalangers, the hallux being opposable to the other toes, 

 and so forming a posterior pair of hands. Their dental formula 

 is I. f , C. \, Pm. |, M. |, x 2 = 50. 



The species of Opossums are very numerous ; but in no family 

 of equal extent are there so few differences in the osteological 

 characters the skulls, teeth, and proportions of the limbs being in 

 all nearly identical. 



The fossil remains of Didelphyida are of special interest, being 

 found in the Eocene deposits of England and France. These 

 fossils consist, however, chiefly of lower jaws, so that it is by no 

 means easy to tell their exact relations to their modern represen- 

 tatives. 



The last family of the Marsupials is the Notoryctida, comprising 

 one single animal only, the Marsupial Mole (Notary ctes typhlops), 

 whose external characteristics have already been referred to (p. 58). 

 The skeleton of this little animal, exhibited in Case 24, Div. C, is 

 remarkable for its generally mole-like structure, powerful fore limb, 

 with its stout and highly ridged humerus, for its ankylosed cervical 

 vertebrse, the first and the seventh being alone free, and for the 

 peculiar roofing in of the sacrum by the expansion of the processes 

 of the sacral vertebrae. The teeth vary slightly in number, but the 

 ordinarv formula appears to be : I. |, C. \, Pm. |, M. f x2 = 40. 



Order XII. MONOTREMATA. 



[Case 24.] The Monotremes (Case 24, Div. C) present many very impor- 

 tant skeletal characters, among which may be specially noticed the 

 peculiar structure of the shoulder-girdle, in which the clavicle is 

 large, and connected with the sternum by an " inter-clavicle ; " the 

 coracoid, instead of being quite rudimentary as in other mammals, 

 is large, and articulates with the sternum; the whole structure 

 being of a very low and reptilian type. The skull is long and 

 depressed, with a large rounded brain-case, the walls of which are 

 thin, as in birds. There are no true teeth in adult life, but in 

 Ornithorhynchus the young are provided with three peculiar flattened 

 saucer-like teeth in each jaw, which are afterwards shed and replaced 



