MONOTREMES, 229 
modification in the usually accepted classification is advis- 
able, there would seem to be considerable justification for the 
removal of the Monotremes from among the Mammals to form 
a separate class by themselves ; although such a radical change 
_ should only be made after much deliberation and careful weigh- 
ing of evidence. 
As characters which may be regarded as distinctive of the 
Monotremes as an Order, in contradistinction to a Sub-Class, 
the following may be noted :— 
In both the two existing families the males are provided with 
a perforated spur on the inner side of the heel, connected with a 
gland behind the thigh, the function of which is not yet 
clearly understood. Although the assumption that this gland 
is poison-secreting has not been proved to be true, it has been 
suggested that during the breeding season if such a function 
| were developed it might be advantageous to the males in their 
conflicts for the possession of the females. 
Like the Marsupials, the Monotremes possess the so-called 
marsupial, or epipubic bones attached to the anterior rim of 
the pelvis. In all the muzzle is produced into a beak, which 
_ May be either flattened or cylindrical ; and true functional teeth 
-are absent at least in the adult. The external aperture of the 
_ €ar opens on the surface of the head without any trace of a pro- 
_ jecting conch ; the short and powerful limbs are of subequal 
length, and adapted for digging; and the tail is either short 
and broad, or rudimental. 
That the existing Monotremes are specialised survivors of an 
extremely ancient stock, is rendered evident on the one hand 
by their peculiar beaks and the absence of teeth in the adult 
or throughout life, and on the other by their low general or- 
ganisation and their marked indications of affinity with the 
lower Vertebrates. On the whole, their nearest allies among 
| the latter seem to be a group of Reptiles whose remains are 
