spines inclined forwards, long, broad, and flat transverse processes pointing forwards and downwards, well developed metapophyses and rudimentary anapophyses.

In the MARSUPIALIA, the number of thoracico-lumbar vertebræ is invariably 19, although there are some apparent exceptions, in which the last lumbar assumes the form of a sacral vertebra. The rib-bearing vertebræ are always 13, except in the Koala (Phascolarctos), which has but II, and one species of Wombat (Phascolomys vombatus), which has 15. The Hairy-nosed Wombat (P. latifrons) has the ordinary number.

In the Kangaroos, the lumbar vertebræ have largely developed metapophyses and anapophyses, and moderatesized transverse processes much curved forwards.

In the running and jumping Bandicoots (Perameles) the lumbar vertebræ have very slender, long, forward-directed spines, and long transverse processes. In the climbing Opossums (Didelphys), on the other hand, the spines are very short and broad from before backwards.

The MONOTREMATA agree with the Marsupials in the total number of trunk vertebræ, but those that bear ribs are more numerous, viz. 16 in Echidna, and 17 in Ornithorhynchus.

The spinous and transverse processes are very short, and the ribs have no articulation with the latter, but are attached to the bodies only, the greater part of the articular surface being below the neuro-central suture, the reverse of what occurs in the higher Mammals. In the thoracic vertebræ the canals for the exit of the spinal nerves perforate the neural arch.

F

CHAPTER VI.

SPECIAL CHARACTERS OF THE SACRAL AND CAUDAL

VERTEBRÆ.

Sacral Vertebra.-The difficulties in defining the sacral vertebræ have been noticed at page 30. Their essential character is best illustrated by tracing it up from the simple condition it presents in the tailed Amphibians (as Menopoma). In these animals a series of similar small straight ribs are movably articulated to the ends of the transverse processes of all the trunk vertebræ, which are not distinctly divisible into separate regions. To the distal extremity of one of these the ilium is attached. This vertebra with its rib thus constitutes the "sacrum," and the ilium is clearly seen not to be a "pleurapophysis," as it is sometimes called, or any part of a vertebra, but a something distinct and superadded. In the Crocodiles there are two vertebræ with strongly developed rib-like bones connecting them to the ilium, and remaining long only suturally united to their vertebræ.

The inferior ossification of the transverse processes of the true sacral vertebræ in Mammals (see Fig. 6, p. 26) is clearly of the same nature, though more rudimentary in character, and coalescing at an earlier period with the remainder of the vertebræ. It is not yet known that it exists in all Mammals,

but this may be considered probable, as it is certainly found, at least in the first sacral vertebra, in such different forms as Man, the Chimpanzee, Orang, Cat, Sheep, Elephant, Sloth and Wombat.

The ankylosis of additional vertebræ in the Mammalia is probably related to the greater fixity and more complete attachment of the pelvis to the vertebral column in this class; for the innominate bone is not only articulated by its iliac portion to the true sacral vertebræ, but it has also a posterior connection with the vertebral column by its ischial portion, by means either of very strong ligaments, or in some cases by bony union.2

In Man there are usually five ankylosed vertebræ, constituting the "os sacrum" of anthropotomy, but only two, or sometimes three, have distinct costal elements. The remainder may be called pseudo-sacral, and belong more properly to the caudal series. The sacrum as a whole is broad, strongly curved in the longitudinal direction, with the concavity downwards, and its anterior extremity forms with the body of the last lumbar vertebra a more prominent "sacrovertebral angle" than in other Mammals.

are

In the Gorilla, Chimpanzee, and Orang, there generally five ankylosed vertebræ, to which the last lumbar not unfrequently becomes united in old animals. The whole sacrum thus formed is long and narrow, gradually tapering posteriorly, and much less curved than in Man. In the other Monkeys, there are usually two or three, rarely four,

"The posterior

1 This is carried to a still greater extent in birds. 2 Hence the following definition of the sacrum : limit of the sacral region is characterized, not by the union of the different osseous pieces, which varies according to age, but by the place of insertion of the ischio-sacral ligaments." (A. MILNE EDWARDS, Famille des Chevrotains, p. 52.)

ankylosed vertebra; the first two, or true sacrals, are broad. and behind these the sacrum suddenly contracts.

In the Lemurina the number of united vertebræ varies

from 2 to 5.

In the CARNIVORA, there are generally few true sacral vertebræ,only 2 in the Puma, Proteles, and Hyena, 2 or 3 in the Tiger; in the Bears and Seals as many as 4 or 5 may be united by bone in old animals. In the Dog there are usually 3 ankylosed vertebræ.

In most UNGULATA and RODENTIA the sacrum consists of one broad vertebra joining the ilia, and a series of narrow ones, varying in number with age, gradually diminishing in width, ankylosed to it behind.

In the Beaver among Rodents, most Chiroptera, the Cape Anteater (Orycteropus) among Edentates, and the Wombat; among Marsupials, the sacrum consists of numerous ankylosed vertebræ, with widely-expanded transverse processes, which are longer in the hindermost vertebræ, and nearly meet the ischia.

In most other EDENTATA, as the Sloths, Anteaters, Armadillos, and in Pteropus among Chiroptera, this modification is carried further, and the transverse processes of the hinder pseudo-sacral vertebræ form a complete bony union with the ischia, converting into a foramen what is usually the sacro-sciatic notch. In some of the Armadillos as many as 10 vertebræ are thus firmly fused together, and with the innominate bones.

In MARSUPIALIA usually but one vertebra supports the iliac bones, though another is commonly ankylosed with it. In the MONOTREMATA, the Ornithorhynchus has 2 ankylosed sacral vertebræ, and the Echidna 3 or 4.

The CETACEA having no iliac bones, have no part of the vertebral column specially modified into a sacrum; but in

the SIRENIA, the rudimentary ilia are attached by ligament to the end of the transverse processes of one vertebra, which may hence be regarded as sacral.

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FIG. 25.-Anterior surface of third caudal vertebra of Leopard (Felis leopardus), §. az anterior zygapophysis; pz posterior zygapophysis; m metapophysis; t transverse process.

Caudal Vertebra.-The vertebræ of the tail vary greatly in number and in characters in different animals. When it is well developed, as, for example, in the long-tailed Carnivora, from one of which the accompanying figures are

FIG. 26-Upper surface of the third caudal vertebra of Leopard, . az anterior zygapophysis; pz posterior zygapophysis; m metapophysis; transverse process.

taken, the anterior vertebræ (Figs. 25 and 26) are comparatively short and broad, with complete neural arches, though without distinct spines, prominent metapophyses, and anterior and posterior zygapophyses (the latter especially