Fig. 51.—Ventral view of skull of Echidna aculeata, and right half of mandible, ang, Angle of mandible; aud.oss, auditory ossicles; cond, condyle of mandible; cor, coronoid process; max, maxilla; oc.cond, occipital condyle; pal, palatine; p.max, premaxilla; pt, pterygoid; sq, squamosal; ty, tympanic ring. (After Parker and Haswell.)
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,
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.[[54]] In the pelvis the acetabulum is perforate (in Echidna), as in Sauropsida.
Fig. 52.—Side view of right half of the shoulder girdle of a young Echidna (Echidna aculeata). × 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.)
Considering the numerous very archaic features which the general structure of this group displays, it is surprising to find how typically mammalian they are in certain other peculiarities. The mammalian diaphragm, one of the distinguishing features of the class, is perfectly normal in the Monotremata. The alimentary canal shows no great divergences from the normal structure. The stomach is almost globular, with a projecting pyloric region in Ornithorhynchus; the intestine is divided into a "small" and "large" intestine by a slender caecum. The liver has the subdivisions that this organ usually shows in the Mammalia. However, the presence of the ventral mesentery and of the abdominal vein in Echidna and Ornithorhynchus has already been mentioned as a distinctive character. The peculiar and apparently partly primitive valve of the right ventricle has been described above (see p. [66]). The brain is in most respects mammalian in its characters, but naturally shows some important differences. Dr. Elliot Smith, who has most recently studied this question,[[55]] is of opinion that the size of the cerebral hemispheres is not at all reptilian; indeed, it "greatly exceeds that of
many other mammals." In Echidna, too, but not in Ornithorhynchus, the hemispheres are well convoluted, though the arrangement of these convolutions cannot be brought into line with what is known concerning the convolutions upon the hemispheres of other mammals. It had been stated that in these animals, at least in Echidna, there were only two optic lobes, as in lower vertebrates, instead of the mammalian four. The late Sir W. H. Flower set this matter at rest,[[56]] and showed that Echidna was in this respect typically mammalian. The absence of the corpus callosum is one of the principal features separating the Monotremes from other mammals.
The Monotremata are represented to-day by two types, Ornithorhynchus and Echidna, which are no doubt worthy of being placed in separate families. Fossil remains of the group (apart from the problematical Multituberculata) are only known from Pleistocene times in Australia, and consist of the bones of a large species of Echidna, and some fragments of Ornithorhynchus, indicating a smaller animal than the living Platypus.
