This sub-class is represented by the single order Monotremata with two families Ornithorhynchidae and Echidnidae. The former contains the single genus Ornithorhynckus found in the rivers of Australia and Tasmania, the latter the genus Echidna found in the same places, as well as in New Guinea, and the genus Pro-echidna from the last-named locality. Ornitho-rhynchus feeds on soft organisms inhabiting mud, etc.: its jaws are shaped like a Duck's bill and covered by thickened epidermis, in which are lodged peculiar tactile organs. The Echnidae have spines mingled with the hairs. They feed on Ants, etc, which they catch with their long extensile tongue. The sub-maxillary gland is of unusual size in this family.

The cranial sutures close, and the surface of the cranium is polished as in Aves. The rami of the lower jaw do not form a symphysis, and have no ascending portion. The cervical ribs are distinct up to a certain age at least: the odontoid process of the axis is long, and not fused to the centrum of that vertebra. There are no epiphyses to the centra of the vertebrae as in Sirenia among Eutheria. There are intermediate ribs as in many Reptilia. The spine of the scapula forms the anterior border of that bone: the coracoids reach the praesternum, and there are large distinct ossified and overlapping epicoracoids and a T-shaped interclavicle. The axis of the ilia is more vertical than usual among Mammals, and the ischia meet in a ventral symphysis of great extent in Ornithorhynchus. The centre of the acetabulum is fibrous in Echidna as in Aves. There are epipubic bones. The fibula has a process homologous with the olecranon. The os calcis is feebly developed, and the males have a curved spur perforated by the duct of a gland, and borne by an accessory ossicle on the inner side of the tarsus.

The spur is present also in the female, but remains rudimentary, and is lost in aged specimens.

The brain has a small corpus callosum and a large anterior commissure. The olfactory nerve quits the skull in Ornithorhynchus as in lower Verte-brata, in a single strand, and there is no lamina cribrosa. The cochlea is curved, not spirally twisted. Ornithorhynchus has eight horny epidermic teeth: the Echidnidae are edentulous. The right auriculo-ventricular valve has no septal flap, or sometimes in Ornithorhynchus a rudimentary one.

There are no chordae tendineae, and the musculi papillares are attached to the membrane of the valve, and in Ornithorhynchus even invade its substance, and extend to the auriculo-ventricular ring; and in this animal a transverse section of the ventricles is remarkably Avian in character 1. The thyroid cartilage in the larynx is formed of two separate cartilages, and the cricoid shows traces of its origin from a number of tracheal rings. The rectal and urogenital canals unite in a common cloaca which is closed by a sphincter muscle. The ureters open below the neck of the bladder into the urogenital canal itself. The testes are retained within the abdomen: the left ovary is larger than the right. The vasa deferentia open separately into the urogenital canal, as do the oviducts. The latter have non-fim-briate abdominal apertures, and are dilated towards their lower extremities with a thickened mucous membrane. There is no thickened muscular uterus. The oviducal aperture opens above that of the ureter on a common papilla. The penis is attached to the ventral wall of the cloaca: and consists of two corpora cavernosa. It is perforated by a canal which can be brought into temporary connection with the openings of the vasa deferentia. The clitoris is large.

The mammary glands are two in number, and their ducts open on an area of the skin which is depressed in Echidna, flat in Ornithorhynchus. The ova are large and meroblastic, and the tunica granulosa of the Graafian follicle consists, as in Sauropsida, of but a single layer of cells. Ornithorhynchus lays its eggs on a rough kind of nest at the bottom of a burrow: Echidna carries them in a marsupial pouch which appears to be developed at stated periods, and to arise in the first instance as two folds, each inclosing a mammary area. The young animal has a knob or caruncle, as in some Aves, on the snout, which probably assists in perforating the tough egg-shell.

The temperature of the body in Echidna is said to be 28°C, in Ornithorhynchus 24.8°C, by Miklucho-Maclay, Nature, xxxi. 1884-85, p. 809. That of other Mammals, according to J. Davy, is 38.4°C.

Gould, 'Mammals of Australia,' 3 vols. 1845-63. Echidna, Oldfield Thomas, P. Z. S. 1885. Corpus callosum of Echidna, Flower, Ph. Tr. 155, 1865. Eye of Ornithorhynchus, Gunn, Journal of Anat. and Physiol, xviii. 1884. Cochlea of same, Pritchard, Ph. Tr. 172, 1881. Tongue of Ornithorhynchus, Poulton, Q. J. M. xxiii. 1883; sense organs of bill in do., Id. Proc. Physiol. Soc. in Journal of Physiology, 1884, p. xv. Heart, Ray Lankester, P. Z. S. 1882; 1883. Reproductive organs, Martin Saint-Ange, Etudes de l'appareil reproducteur, Paris, 1854. Ovum, Poulton, Q. J. M. xxiv. 1884; of Echidna, Beddard, Proc. Roy. Phys. Soc. Edinburgh, viii. 1885; Ramsay, A. N. H. (5) xvi. 1885; Do. and oviparity, Baldwin Spencer, Nature, xxxi. 1884-85. Marsupial ovum, pouch and mammary glands of Echidna, Haacke, P. R. S. xxxviii. 1884-85, cf. von Lendenfeld, Z. A. ix. 1886.

1 An anterior abdominal or epigastric vein, arising from the bladder and distributed to the left lobe of the liver, has been found in a female Echidna. It may not be a constant structure. Beddard, P. Z. S. 1884.