EGG-LAYING MAMMALS. 215 



the skeleton known as the shoulder-girdle, or that serving for the support and 

 articulation of the bones of the fore-limb. In all the higher mammals, with 

 the exception of some of the Edentates (where the coracoids may be distinct), 

 the shoulder-girdle comprises, at most, only a pair of collar-bones (clavicles) and 

 two shoulder-blades {scapulce), of which the latter bear a so-called coracoid 

 process at the lower extremity. On the other hand, in the egg-laying mammals, 

 as in lizards, an unpaired additional T-shaped bone known as the inter- 

 clavicle overlies the collar-bones and breast-bone (sterimm); while the coracoid 

 processes are represented by a pair of separate coracoid bones situated on the 

 lower surface of the chest, and connected with the blade-bones to form the 

 articular cavity for the head of the upper arm-bone, or humerus. Behind 

 these, on the same aspect of the body, are a second pair of bones known as 

 the meta-coracoids, and representing the bones commonly called coracoids in 

 birds, which are articulated inferiorly to the breast-bone. The brain is also 

 of a lower and simpler type than in the higher mammals ; the same being 

 also the case with the small b^nes of the internal ear. Another feature 

 (which at present appears to have been verified only in the first of the two 

 f'lmilies) indicative of inferior organisation is the low temperature of the 

 blood as compared with that of other mammals. 



In both the families of Monotremes the males are furnished on the 

 inner surface of the heel with a perforated horny spur, connected with a 

 gland behijyi the thigh ; and it is not improbable that during the breeding 

 season this gland may secrete a poisonous fluid which flows into any 

 wound inflicted by the spur. As in the Pouched Mammals, the Monotremes 

 have a pair of marsupial bones attixed to the front edge of the lower part 

 of the pelvis. Both families have the skull produced into a beak, which 

 may be either cylindrical or flattened ; and in the adult state at least, teeth 

 are wanting. There are no traces of external ear-conohs ; the two pairs of 

 limbs, which are of nearly equal length, are modified for burrowing or 

 digging ; and the tail ia either rudimental or short and wide. As in the case 

 of the Edentates, there is no clue to the origin of the Monotremes, but there 

 can be no doubt that the existing forma are highly modified and specialised 

 descendants from some extremely ancient stock. 



The strange Australian creature, known as the duck-bill or duck-mole 

 (OriiiiAo?7ii/)ic^Msaiici<wiMs), is thesole representative of the firstfamily(Oraii7io- 

 rhynchidce) of the Monotremes. As distinctive of the family, 

 the following features appear the most important : — In the Duck-bill, 

 head the muzzle is produced and expanded to form a broad, 

 depressed beuk, covered with a delicate sensitive skin, which, in the dried 

 state, assumes the appearance of horn. The tongue is of normal proportions, 

 and non-extensile. In the fur there is no mixture of spines ; the tail, although 

 short, is broad and flattened ; and the feet are largeand wide, with the toes 

 webbed, and the soles bare and without pads. In the young and adolescent 

 condition, each jaw bears three pairs of many-cusped molars, of which the 

 first is the smallest in the upper jaw, and the last in the lower ; but in the 

 adult these teeth are shed, and their functions discharged by horny plates 

 growing up around and beneath them. In the male the horny spur on the 

 hind-foot is very large ; while in the female the pouch is rudimental. The 

 brain has smooth hemispheres. The duck-bill has a somewhat elongated 

 and depressed body, covered with dark-coloured fur of two kinds. In both 

 limba the five-toed feet have long claws to all the digits, although those on 

 the fore-feet are broad and blunt, while those behind are compressed and 



