(2181). From this analysis of the composition of the cranium, it is apparent that the temporal bones, although in Man they assist so materially in completing the cranial cavity, are only intercalated between the real vertebral elements; as indeed might almost have been anticipated, seeing how differently the pieces belonging to this bone are arranged in different classes of Vertebrata.

(2182). Such is the general organization of the Mammiferous skeleton. Let us now proceed to consider the osteology of the different orders into which the Mammalia have been distributed, and observe in what respects they individually differ from each other.

(2183). The transition from Birds to Quadrupeds, remotely separated as they might appear to be, is effected by gentle gradations of structure; and the Mono-tremata, notwithstanding their quadrupedal form and hairy covering, are so nearly allied to the feathered Ovipara in many points of their organization, that they evidently form a connecting link between these two great classes of animals.

(2184). It is true that they have mammary glands, and must therefore be supposed to give suck to their offspring; but it is not even yet satisfactorily determined whether they lay eggs, or produce living young. The structure of their generative apparatus would seem, in fact, to be rather allied to the Oviparous than the Mammiferous type; and, as in Birds, the rectum, the urinary passages, and the sexual organs, all discharge themselves into a common cloacal chamber; so that there is still but a single vent - a circumstance from which the name of the order is derived.

(2185). Even their skeleton, in many points, presents a very close affinity to that of a Bird, as will be evident on examining the osseous system of the Ornithorhynchus paradoxus (fig. 383).

(2186). The mouth of this quadruped, indeed, resembles that of a Duck, whence the name of "Duck-bill," whereby it is usually distinguished. It has, moreover, a distinct furcular bone in addition to what would seem to be the ordinary clavicles; but in reality these are the coracoid bones still largely developed. Moreover, the anterior or sternal ribs are ossified, and a spur is attached to the hind foot of the male, not remotely resembling that of a Cock: this last appendage is perforated by a duct, and has a gland connected with it, situated on the inner side of the thigh, by which a poisonous secretion was formerly supposed to be elaborated.

Cranial vertebrae.

Fig. 382. Cranial vertebrae.

Skeleton of Omithorynchus paradoxus.

Fig. 383. Skeleton of Omithorynchus paradoxus.

(2187). The Marsupialia, it will be afterwards explained, as regards the conformation of their generative system, are organized in accordance with a type intermediate between that common to Birds and that which characterizes Mammalia properly so called.

(2188). The Marsupial quadrupeds bring forth their young alive, but in such an imperfect condition, that at the period of their birth scarcely the rudiments of their limbs have become apparent; and in this state they are conveyed into a pouch formed by the skin of the female's abdomen, where they fix themselves by their mouths to the nipples of their mother, and, sucking milk, derive from this source the materials for their growth. These animals are peculiar to the Australian and American continents; nay, in Australia, so anomalous in all its productions, with one or two exceptions, and those perhaps brought there by accidental importation, all the quadrupeds are constructed after the Marsupial type. The great characteristic whereby to distinguish the skeleton of a Marsupial Mammifer is, the existence of two peculiar bones attached to the anterior margin of the pubis, which in the living animal are imbedded in the muscular walls of the abdomen, and thus support the pouch of the female. The marsupial bones, however, exist in the male likewise; and even in the Monotremata, that are evidently nearly allied to the proper Marsupialia, although no pouch is met with even in the female sex, the bones alluded to are found connected with the pubis.

(2189). This great section of the Vertebrate creation, which perhaps ought rather to be regarded as a class by itself, is composed of numerous families, of diverse forms and very opposite habits. The Opossums (Didelphys) of the American continent live in trees, and devour birds, insects, or even fruits: in these, the thumb of the hind foot is opposable to the other fingers, and adapted for grasping the boughs, whence they are called Pedimanes; their tail is likewise prehensile. Others are terrestrial in their habits, wanting the prehensile thumb.

Skeleton of the Kangaroo Rat.

Fig. 384. Skeleton of the Kangaroo Rat.

(2190). The Kangaroo Rat, or Potoroo (Hypsiprymnus), of whose skeleton we have given a drawing (fig. 384), is remarkable for the disproportionate size of its hind legs: these, moreover, have no thumb, and the two innermost toes are joined together as far as the nails; so that there appear to be but three toes, the inner one being furnished with two claws. Such legs are well adapted to make strong and vigorous leaps over a level plain; and in the Kangaroos (Macropus) the extraordinary development of the posterior extremities is even yet more wonderful. In other respects, the skeletons of the Marsupialia conform to the general description already given.

(2191). All other MammiferousVortebrata produce their young alive, and not until they have attained a considerably advanced state of development during their intra-uterine existence. The connexion between the maternal and foetal systems in these orders is maintained during the latter periods of gestation by the development of a peculiar viscus, called the placenta; nevertheless, after birth, the young animals are still dependent upon the mother for support, and live upon the milk supplied by her mammary organs.