Of the vertebral column.—Both Monotremes have twenty-six true vertebrae, of which the seven first are cervical. In the Echidna sixteen, and in the Ornithorhynchus seventeen, of the following vertehrm support long and functional ribs, so that there are three lumbar vertebra' in the Echidna, and but two in the Ornithorhyn chus, which thus resembles the Lizards in the great proportion of the trunk which is encom passed by the costal arches. Another approxi mation to the Oviparous type is made by the long-continued separate state of the short cervical ribs in both Monotremes : these in a young but nearly full-grown Echidna are detached from all the cervical vertebra; except the atlas. The vertebral end of the cervical rib is ; the lower branch, represent ing the head, is articulated to the transverse process or tubercle developed from the body of the vertebra ; the upper branch, represent ing the costal tubercle, is articulated to a trans verse process developed from the side of the base of the neural arch. In the Ornitho rhynchus the cervical ribs appear to become earlier anchylosed to the vertebra, except as regards the axis, in which the broad costal appendage retains its original independence throughout life, and is slightly moveable upon the confluent extremities of the two transverse processes. In the succeeding ver tebra the space intercepted between the two transverse processes, the vertebra, and the cos tal rib, forms the so-called ' perforation of the transverse process' for the vertebral artery in human anatomy. In the Echidna, above alluded to, the ' neurapophyses' or vertebral plates of the atlas, which together form the neural or spinal arch, were unanchylosed at their upper or spinal extremities. The atlas of the Orni thorhynchus is chiefly distinguished from that of the Echidna by the continuation, from the lower part of its slender body, of two long diverging processes which are developed in the strong tendons of the recti capitis antici muscles. The spine of the dentata is broad and high in both Monotremes; those of the other cervicals progressively diminish in size in the Ornithorhynchus, but become at once nearly obsolete in the Echidna. The transverse and spinous processes are of moderate size in the rest of the true vertebra, but are largest in the lumbar region. The posterior oblique processes are single in these vertebra. The articular surfaces of the vertebra, which are slightly concave, are joined together by a thick circular band of ligamentous fibres (fig. 174, a) attached to the circum ference of the articular surface, enclosing a cen tral oblate spheroidal ca vity (b) lined by a synovial membrane and filled with fluid.
The ribs are long and slender in the Ornitho rhynchus, somewhat stron ger in the Echidna. The first is flattened, the rest are cylindrical. Each rib is articulated by a single joint, uniting the head to the vertebral. interspace, or to the side of the cen trum, as in the two last pairs : the tubercle, though small, is distinctly developed, and de fines the neck of the rib, although it does not join the transverse process of the vertebra.
Meckel's statement, tuberculum adest nul lum,' is applicable only to the last three or four pairs of ribs. The first six pairs are joined to the sternum in both the Ornithorhynchus and the Echidna. Six pairs of ossified sternal ribs (hwmapophysess) are articulated to the ster num in the Echidna; the first four are nearly straight and sub-cylindrical; the fifth and sixth are expanded. Five pairs of ossified sternal ribs are present in the Ornithorhynchus, to which the second to the sixth vertebral ribs inclusive are joined by shorter intervening cartilaginous pieces of a similar form. The
first rib in the Ornithorhynchus is joined to the sternum by cartilage alone. The interposed cartilages, which thus form a third element in the costal arch, repeat a structure common in crocodiles, and may be regarded as the homo logues of the costal appendages in the ribs of birds, which in this class are removed from the interspace of the vertebral and sternal rib, and articulated to the vertebral piece.
The cartilages of the false ribs in both genera are singularly expanded and flattened, and present an imbricated arrangement, gliding upon each other with a slight yielding motion : the last vertebral rib, which is the shortest and straightest, is terminated by a short free carti lage. Many bone-corpuscles are scattered through these cartilages.
The ordinary sternum, to which the true ribs articulate, consists of four ossicles, and in the Echidna a fifth is developed in the base of the ensiform cartilage ; the most anterior of these oisicles (fig. 173, A, s) has the usual expanded hexagonal form and large proportions of the manubrium sterni, and receives, besides the first and part of the second pair of ribs, the extremities also of the coracoid bones. The ensiform cartilage in the Echidna presents an elongated oval form with a central perforation. A large T-shaped episternal bone (fig. 173, A, t) is articulated to the anterior surface of the manubrium sterni ; it is the key-bone of the complicated scapular arch.
The sacrum consists in the Ornithorhynchus, as in most Saurians, of two distin guished by the greater breadth and thickness of their transverse costal processes. In the Echidna there are three sacral vertebra.
There are thirteen caudal vertebra in the Echidna (fig. 168). The first is the largest, with broad transverse processes, the rest pro gressively diminishing, and reduced, in the six last, to the central element. The Ornithorhyn chus (fig. 173, A) has twenty-one caudal ver tebra, of which all but the last two have transverse processes, and the first eleven have also spinous and articular processes. The transverse processes are broad and depressed ; they gradually increase in length to the tenth caudal, then as gradually diminish to the twen tieth ; their extremities are expanded, and, from the fifth backwards, are thickened and tuberculate. The spinous processes progres sively diminish in height from the first caudal. The first six caudal vertebrae have both posterior and anterior oblique processes, and are joined together both by these and by the articular surfaces of the body : the anterior articular processes are present, but progres sively diminish in size from the seventh to the sixteenth vertebrae, and are not subservient to their reciprocal articulation. Inferior spinous processes are developed from the bodies of the third to the nineteenth caudal vertebra inclusive; but there are no limmapophyses articulated to the vertebral interspaces, as in many Marsupials and the Edentata. In the Echidna the inferior spinous processes are absent ; but rudiments of limmapophyses are connected with the interspaces of one or two of the middle vertebrae of the tail. The cau dal vertebrae in the Ornithorhynchus are of nearly the same length to the two last; they progressively diminish in vertical diameter as they recede from the trunk, and are chiefly remarkable for their breadth and flatness; resembling in this respect, as Cuvier has ob served, the caudal vertebra of the Beaver, and we might add those of the Cetacea; the hori zontally extended tail having a similar relation to the frequent need which an aquatic animal with hot blood and a quick respiration of air has to ascend rapidly to the surface of the water.