102 
OSTEOLOGICAL GALLERY. 
in their dentition the Dasijvrida;, 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. C. Pm. ]\1. 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 Didelphyidce 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 Notoryctidee, comprising 
one single animal only, the Marsupial Mole [Notoryctes 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 
vertebrjc, 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: — 1. C. Pm. §, M. | x 2 = 40. 
Order XIL MONOTREMATA. 
:.j 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 tyj)e. 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 
