12 GENERAL ANATOMICAL CHARACTERS 
Equide, the grasping under surface of the tail of the prehensile-tailed 
Monkeys, etc. The greater part of the skin of both species of 
one-horned Asiatic Rhinoceros is immensely thickened and stiffened 
by increase of the tissue both of the derm and epiderm, con- 
stituting the well-known jointed “armour-plated” hide of those 
animals. 
Nails, Claws, and Hoofs.—With very few exceptions, the terminal 
extremities of the digits of both limbs are more or less protected or 
armed by epidermic plates or sheaths, constituting the various forms 
of nails, claws, or hoofs. These are wanting in the Cetacea alone. 
A perforated spur, with a special secreting gland in connection with 
it, is found attached to the hind leg of the males of the three genera 
of Monotremata, Ornithorhynchus, Proechidna, and Echidna, 
Odour - secreting Glands.—Besides the universally distributed 
sebaceous glands connected with the pilose system, most mammals 
have special glands situated in modified portions of the integument, 
often involuted to form a shallow recess or a deep sac with a narrow 
opening, situated in various parts of the surface of the body, and 
secreting odorous substances, by the aid of which individuals 
appear to recognise one another, and probably affording the princi- 
pal means by which wild animals are able to become aware of 
the presence of other members of the species, even at great dis- 
tances. Although the commencement of the modifications of 
portions of the external covering for the formation of special 
secretions may be at present difficult to understand, the principle 
of natural selection will readily explain how such organs become 
fixed and gradually increase in development in any species, especi- 
ally as there would probably be a corresponding modification and 
increased sensibility of the olfactory organs. Such individuals as 
by the intensity and peculiarity of their scent had greater power of 
attracting the opposite sex would certainly be those most likely to 
leave descendants to inherit and in their turn propagate the modi- 
fication. 
To this group of structures belong the suborbital gland or 
“crumen” of Antelopes and Deer, the frontal gland of the Muntjak 
and of Bats of the genus Hipposiderus, the submental gland of the 
Chevrotains and of Taphozous and some other Bats, the post-auditory 
follicle of the Chamois, the temporal gland of the Elephant, the 
lateral glands of the Musk-Shrew, the dorsal gland of the Peccary, 
the inguinal glands of Antelopes, the preputial glands of the Musk- 
Deer and Beaver (already alluded to in connection with the use 
made of their powerfully odorous secretion in medicine and per- 
fumery) and also of the Swine and Hare, the anal glands of Carni- 
vora, the perineal gland of the Civet (also of commercial value), the 
caudal glands of the Fox and Goat, the gland on the humeral 
membrane of Bats of the genus Saccopteryx, the post-digital gland of 
