FORE (A) AND HIND (B) FOOT OF THE DUCK-BILLED PLATYPUS.
(After Waterhouse.)
(a) Spur on Heel of Hind Foot.
SHOULDER-GIRDLE AND STERNUM OF THE ECHIDNA.
(a) T-shaped interclavicle; (b) Manubrium; (c) Ensiform end of sternum; (d) Cartilages of ribs; (e) Scapula; (f) Coracoid; (g) Epicoracoid.
It was thought that this bird-like creature laid eggs, but the point was not easy to determine. (See Postscript, page 234.) It has a double uterus, leading to the common canal, called urogenital, and this ends in the common outlet. The Ornithorhynchus and Echidna have an arrangement of the bones of the shoulder and chest, which resembles to a certain extent that of the Lizards and of the Ichthyosaurus, and the annexed engraving will explain the position of the bones. Indeed, the most important peculiarity in the skeleton of the Monotremes is that of the shoulder-girdle and upper part of the chest; for a bone, the merest vestiges of which are noticed in some of the Mammalia, occurs, that is of some importance in the great groups of birds and reptiles, which are lower in the animal scale than the Vertebrata already described. In all the animals described hitherto, and including the Marsupiata, the large arm bone (humerus) is jointed at the shoulder with the blade bone, or scapula. The socket in this bone, which receives the somewhat ball-shaped top of the humerus, in order to permit of very general motion, is a part of the scapula, and is called the glenoid cavity; but in the Monotremes a bone called the coracoid joins with the scapula, and forms part of the socket; moreover, this coracoid is long enough to reach the breast bone, or sternum. The breast bone in the Mammalia hitherto noticed consists of an expanded part at its fore end (in the usual position of quadrupeds), or at its top in man, called the manubrium, and of some smaller pieces, which form the front bone of the chest and reach to the belly, having ribs attached to them on each side. This is the state of things in the Monotremes; and the coracoids are attached to the manubrium, one on each side. In other Mammalia it is the collar bone which is jointed there. In addition to these breast bones in the Monotremes, there are other bones in front, or between the neck and the top of the manubrium. Firstly, there is a bone in the shape of a T: the lower point is on the breast bone, and the cross-bar supports a collar bone on each side, which reaches outwards to the blade bone. Secondly, there is a bone on each side in front of the coracoid, reaching forwards towards the neck. This is called the epicoracoid. Some of these bones, now noticed for the first time, are more or less common to birds, reptiles, and amphibians.
DUCK-BILLED PLATYPUS.
There are some other anatomical points which ally the Monotremes to the reptiles. For instance, the peg on the second, or axis vertebra of the neck, is not fixed to the bone by true bony matter, and some of the ribs which exist in the neck in the Monotremes are separate from the vertebræ until late in life, or altogether. And the cavity for jointing of the thigh bone with the pelvis (the acetabulum) is not perfect, there being a part of it not filled with bony matter.