CHAP. XIII.] THE GAT'S PLACE IN NATURE. 469 



rats, mice, squirrels, guinea-pigs, porcupines, beavers, jerboas, rat- 

 moles, and rabbits and hares. Some rodents are, like the flying 

 opossums, fitted to flit through the air in long jumps, by means of 

 the wide extensibility of the skin of their flanks, which, when 

 stretched out, acts as a parachute — such are the so-called " flying- 

 squirrels " and the genus Anomahcnis. 



Lastly we have the order to which all the kinds of cats belong, 

 together with all pole-cats, civet-cats, hyaenas, bears, weasels, dogs, 

 wolves, and foxes — the order of flesh-eating mammals |jar excellence, 

 or Carnivora. 



These various orders of the class Mammalia are grouped together 

 into three sub-classes as follows : — 



The order Monotrcmata is an order so exceedingly difl'erent from 

 all the others that there can be no doubt (in spite of the few species 

 it contains) of its forming a sub-class by itself. It forms the sub- 

 class Ornithodelphia. 



The order Marsnjyialia difiers much less from the great bulk of 

 mammals than does the order Monotreniata, nevertheless it also is 

 reckoned as forming a second sub- class by itself. It forms a sub- 

 class much more numerous than the sub-class Ornithodelphia, yet it 

 contains but very few species when compared with the number of 

 species contained in the third sub-class. The second or Marsupial 

 sub-class bears the name Didelphia. All the rest of the mammalia — 

 that is to say the whole of the orders Ungulata, Prohoscidea, Sirenia, 

 Cetacea, Pinnipedia, Primates, Cheiroptera, Edentata, Imectivora, 

 Rodentia, and Carnivora, together make up the third and last 

 mammalian sub-class, that containing the placental mammals — the 



sub-class MONODELPHIA. 



§ 19. Now the cat, as a monodelphous mammal, of course shares 

 the characters which distinguish that sub-class from both the others. 

 "We have then first to see how the cat thus difi'ers from the 

 Ornithodelphia. 



The Ornithorhynchus and Echidna agree together in having an 

 exceedingly rudimentary corpus callosum, a condition perhaps more 

 or less compensated for by the very large size of the anterior com- 

 missure. In the internal ear, the cochlea instead of being spirally 

 coiled, as we saw it to be in the cat, is but slightly twisted. The 

 vertebrae are not ossified by the intervention of epiphyses, and the 

 shoulder-girdle is of remarkable complexity. Instead of mere 

 coracoid processes, such as we saw in the cat, there are large 

 coracoid bones (as in birds) with epicoracoid bones, and an inter- 

 clavicle — as in many lizards. The acetabula are also perforated (as 

 in birds) and the fibula sends up a large olecranon-like process, 

 rendering its serial homology with the ulna strikingly evident. 

 The internal tendon of the external oblique muscle of the abdomen 

 is ossified, resulting in the development of two large bones, called 

 " Marsupial,'' which are attached to the front margin of the pelvis. 

 The mouth is either edentulous, or if, as in the Ornithorhynchus, 

 there are teeth, then these are not calcareous but of a horny 



