Page images
PDF
EPUB

lous, and the posterior tetradactylous, the two external digits being nailless.

Megalonyx, unlike the preceding, has been found in both North and South America. It has the same number of teeth as Megatherium and Mylodon, but the crowns of these are excavated centrally and have a prominent margin. The fore-limbs are shorter than the hind-limbs, and the calcaneum is unusually long. Megalonyx was probably about the size of an ox.

Just as the Sloths of the present day were formerly represented in the same geographical area by the gigantic Megatheroids, so the little banded and cuirassed Armadillos of South America were formerly represented by gigantic species, constituting the genus Glyptodon. The Glyptodons (fig. 345) differed from the living Armadillos in having no bands in their armour, so that they must have been unable to roll themselves up. It is rare at the present day to meet with any Armadillo over two or three feet in length; but the length of the Glyptodon clavipes, from the tip of the snout to the end of the tail, was more than nine feet.

[graphic][subsumed]

Fig. 345.-Glyptodon clavipes. Post Pliocene, South America.

There are no canine or incisor teeth in the Glyptodon, but there are eight molars on each side of each jaw, and the crowns of these are fluted and almost trilobed. The head is covered by a helmet of bony plates, and the trunk was defended by an armour of almost hexagonal bony pieces united by sutures, and exhibiting special patterns of sculpturing in each species. The tail was also defended by a similar armour, and the vertebræ were mostly fused together so as to form a cylindrical bony rod. The feet are massive, and the ungual phalanges short and compressed.

Besides the various species of Glyptodon, South America has also yielded the remains of several true species of Dasypus, nearly allied to the living Armadillos. These have been found

in the ossiferous caverns of Brazil; in which occur also other Edentates, which have been referred to separate genera, but the affinities of which are somewhat dubious.

CHAPTER XXXVII.

ORDERS OF MAMMALIA—Continued.

ORDER IV. SIRENIA. This order comprises no other living animals except the Dugongs and Manatees, which are often placed with the true Cetaceans (Whales and Dolphins) in a common order. There is no doubt, in fact, but that the Sirenia are very closely allied to the Cetacea, and though they are to be regarded as separate orders, yet they may be advantageously considered as belonging to a single section, which has been called Mutilata, from the constant absence of the hindlimbs.

The Sirenia agree with the Whales and Dolphins in their complete adaptation to an aquatic mode of life (fig. 346); especially in the presence of a powerful caudal fin, which differs from that of Fishes in being placed horizontally and in being a mere expansion of the integuments, not supported by bony rays. The hind-limbs are wholly wanting, and there is no sacrum. The anterior limbs are converted into swimming-paddles or "flippers." The snout is fleshy and well developed, and the nostrils are placed on its upper surface, and not on the top of the head, as in the Whales. Fleshy lips are present, and the upper one usually carries a moustache. The skin is covered with fleshy bristles. The head is not disproportionately large, as in the true Whales, and is not so gradually prolonged into the body as it is in the latter. There may be only six cervical vertebræ. The teats are two in number and are "thoracic," -i.e., are placed on the chest. There are no clavicles, and the digits have no more than three phalanges each. The testes are retained throughout life within the abdomen, but vesiculæ seminales are present. The animal is diphyodont, the permanent teeth consisting of molars with flattened crowns adapted for bruising vegetable food, and incisors, which are present in the young animal, at any rate. In the extinct Rhytina it does not appear that there were any incisor teeth.

The only existing Sirenia are the Manatees (Manatus) and

the Dugongs (Halicore), often spoken of collectively as "seacows," and forming the family of the Manatida.

Fig. 346.-Sirenia. Dugong (Halicore).

The most important, if not the only, fossil remains which can be referred with certainty to the Sirenia, are those upon which the genus Halitherium has been founded. The upper incisors in this genus are tusk-like, the lower incisors small, and the molars furnished with tubercular crowns. Halitherium appears to be in some respects intermediate between the Dugongs and Manatees; and several species of the genus are known, ranging from the Eocene to the Pliocene Tertiary.

The genus Deinotherium referred to this order by De Blainville, and still retained in this position by Pictet, will be here considered as belonging to the order of the Proboscidea.

ORDER V. CETACEA.-In this order are the Whales, Dolphins, and Porpoises, all agreeing with the preceding in their complete adaptation to an aquatic life. The body is completely fish-like in form; the anterior limbs are converted into swimming-paddles or "flippers;" the proximal bones of the fore-limbs are much reduced in length, and the succeeding bones are shortened and flattened, and are enveloped in a tendinous skin, thus reducing the limbs to oar-like fins; there are no external ears; the posterior limbs are completely absent; and there is a powerful, horizontally-flattened, caudal fin, sometimes accompanied by a dorsal fin as well. In all these characters the Cetacea agree with the Sirenia, except in the one last mentioned. On the other hand, the nostrils, which may be single or double, are always placed at the top of the head, constituting the so-called "blow-holes" or "spiracles ;" and they are never situated at the end of a snout. The body is very sparingly furnished with hairs, or the adult may be completely hairless. The teats are two in number and are placed upon the groin. The head is generally of disproportionately large size, and is never separated from the body by any distinct constriction or neck. The lumbar region of the spine is long,

and, as in the Sirenia, there is no sacrum, and the pelvis is only present in a rudimentary form. There are no clavicles, and some of the digits may possess more than three phalanges each. Lastly, the adult is either destitute of teeth or, with exception of the Zeuglodonts, is monophyodont-that is to say, possesses but a single set of teeth, which are never replaced by others. When teeth are present, they are usually conical and numerous, and they are almost always of one kind only.

The Cetacea may be divided into the two sections of the Balanida, comprising only the "Whalebone Whales," in which true teeth are absent; and the "Toothed Whales" or Odontoceti, comprising the living families of the Delphinida (Dolphins and Porpoises), the Catodontida (Sperm Whales), and the Rhynchoceti ("Ziphioid" Whales), with the extinct family of the Zeuglodontida.

Fig. 347.-Skull of the Right Whale (Balana mysticetus)-after Owen.

Fam. 1. Balanida.-The Balanida or Toothless Whales are characterised by the total absence of teeth in the adult (fig. 347). Teeth, however, are present in the foetal Whale, but they never cut the gum. The place of teeth is supplied by a number of plates of whalebone or "baleen" attached to the palate; hence the name of "Whalebone Whales" often given to this family. They are the largest of living animals, and may be divided into the two sections of the Smooth Whales, in which the skin is smooth, and there is no dorsal fin (as in the Greenland Whale), and the Furrowed Whales, in which the skin is furrowed, and a dorsal fin is present (as in the so-called Finner Whales and Hump-backed Whales).

The Balanida are of little geological importance. In Pliocene deposits have been found remains referred to the Rorquals; and bones of the Whalebone Whales have also been found in various Post-Tertiary accumulations. It is probable, like

[ocr errors]
[graphic]

wise, that the ear-bones or "cetotolites" which occur in the Red Crag (Pliocene) are, in some instances at any rate, referable to members of the Balanidæ.

Fam. 2. Catodontida.-The family of the Catodontida, or Physeterida, comprises the Sperm Whales or Cachalots, with which we commence the series of the Toothed Whales (Odontoceti). They are characterised by the fact that the palate is destitute of baleen plates, and the lower jaw possesses a series (about fifty-four) of pointed conical teeth, separated by intervals, and sunk in a common alveolar groove, which is only imperfectly divided by septa. The upper jaw is also in reality furnished with teeth-but, with a single partial exception, these do not cut the gum.

Remains of Cachalots (Physeter) occur in the Pliocene and Post-Tertiary deposits, and their existence has even been indicated in the Miocene Tertiary. They are, however, of no special importance.

Fam. 3. Delphinida.-This family includes the Dolphins, Porpoises, and Narwhal, and is characterised by usually possessing teeth in both jaws; the teeth being numerous, and conical in shape. The nostrils, as in the last family, are united, but they are placed further back, upon the top of the head. The single blow-hole or nostril is transverse and mostly crescentic or lunate in shape. The head is by no means so disproportionately large as in the former families, usually forming about one-seventh of the entire length of the body.

Fig. 348.-The common Dolphin (Delphinus delphis).

The genus Delphinus, comprising the common Dolphins, appears to date from the Miocene Tertiary, and is well represented in deposits of Pliocene age. In Miocene strata also occur the Delphinoid remains which have been referred to the genera Stereodelphis and Champsodelphis.

Fam. 4. Rhynchoceti.-This family is allied to the Cachalots or Sperm Whales, and includes the so-called "Ziphioid Whales." They are distinguished by the possession of a

« PreviousContinue »