Mammalia, Monotremata. 65 



in the Saltatory species the disproportion in the development of 

 the bones of the hind leg is very great. In the Saltatory species 

 the Fibula is in close contact with or attached to the Tibia, so as 

 to ensnre fixity and strength : in others the Tibia and Fibula are so 

 loosely connected together, and with the Tarsus, that the foot enjoys 

 a movement of rotation analogous to that of the hand. A 

 degeneration of the Foot, common to many Marsupials, is to bo 

 observed in the slender condition of the second and third toes, as 

 compared with the fourth and fifth, and their enclosure nearly to 

 their extremities in a common integument : in the Saltatorial genera 

 they are reduced to almost filamentary slendorness. This condition 

 of the Pes called syndactylism prevails in all Marsupials, except the 

 Didelphidee, Dasyuridse, and Phascolomyidae. In most of the Plant- 

 eating Marsupials no rudiment of the innermost toe exists. 



XV. MONOTREMATA. 



21w Skull. The Monotremata differ from the Marsupialia in the 

 ahsence of the inflected process developed from the angle of the lower 

 Jaw. The Skull is long and low, the facial bones projecting in a 

 more or less beak-like form. The Sutures between the cranial bones 

 are eatly oUiterated. The Frontal bones expand as they rise, but do 

 not develope supra-orbital ridges. The Malar and Squamosal bones 

 are confluent (unless the slendqr process of the maxillary may repre- 

 sent the malar). The Lachrymals, also, are confluent with adjacent 

 bones. The Pterygoid processes of the Sphenoid often remain sepa- 

 rated throughout life. The Pterygoids are flattened, horizontal, 

 oval plates, attached to the obliquely truncated postero-external 

 extremities of the Palatines, and form part of the floor and the 

 inner wall of the tympanic cavity, an arrangement not met with in 

 any other Mammal. The Stapes is columelliform ; one cms of the 

 incus anchyloses with the reduced tympanic, the other is confluent 

 with the malleus. In many of these points the Ornithic affinity of 

 this order is apparent. In no Mammal is the proportionate size of 

 the lower Jaiv as compared with the skull so small as in Echidna, in 

 which it consists of two long styliform rami, loosely connected 

 anteriorly, and bound up with the superior mandible in a continuous 

 sheath of skin, except just at the apex. The lower jaw of Ornitho- 

 rhyncus, on the contrary, resembles very much a flattened duck's 

 bill ; it consists of two horizontal rami, united in the middle of 

 their length, diverging anteriorly and posteriorly from that point, 



