The skeleton shows numerous ancient characteristics. In the skull there is no demarcation of the orbit from the temporal fossa, a feature widely found in archaic mammals. The tympanic remains as a slender ring, there being no auditory bulla formed either from this or from any other bone. The malleus and incus are large, and thus reminiscent of the quadrate and articular bone of reptiles. In the lower jaw the absence of a marked coronoid process, and the absence of a firm ossification at the meeting of the two rami, may be a primitive state of affairs. It must be remembered, however, that the Cetacea show the same characters, though it is possible that they too are developed from a low mammalian stock. In the vertebral column we find the typical mammalian seven cervicals; but those characteristically mammalian structures the epiphyses are totally absent in _Echidna_, and only to be seen in the tail-region in _Ornithorhynchus_. In having only the capitular head to the ribs, these mammals are evidently far removed from all other mammals, and are even more reptilian than the Theromorphous reptiles. The large clavicles and the interclavicle (Fig. 52, {109} p. 109) are characteristic of the group, and the latter bone is peculiar to the Monotremata among mammals. So, too, is the large coracoid. In the scapula there is a spine which coincides with the anterior border of that bone. The arrangement of the muscles in this region proves conclusively that this projection is the homologue of the spine and the acromion of other mammals. Here, again, we have a point of likeness to the Cetacea.[56] In the pelvis the acetabulum is perforate (in _Echidna_), as in Sauropsida. [Illustration] FIG. 52.--Side view of right half of the shoulder girdle of a young Echidna (_Echidna aculeata_). x 1. _a_, Acromion; _c_, coracoid; _cb_, coracoid border; _cl_, clavicle; _css_, coraco-scapular suture; _ec_, epicoracoid; _gb_, glenoid border; _gc_, glenoid cavity; _ic_, interclavicle; _pf_, postscapular fossa; _ps_, presternum; _s_, spine; _ss_, suprascapular epiphysis; _ssf_, subscapular fossa. (From Flower's _Osteology_.)