MONOTREMATA. 527 



The mammary glands consist of two groups of glandular tubes which 

 open in a tuft of hairs on two, slightly less hairy, patches of the skin of 

 the abdomen, by numerous openings. The gland-tubes are of the sudori- 

 parous type, being dichotomously branched and pro\dded with smooth 

 muscular fibres outside their epithelium. In Echidna, but not in Ornitho- 

 rhynchus, these mammary areas are at the breeding season sUghtly de- 

 pressed and placed on the side walls of a forwardly opening pouch of the 

 ventral integument. The egg is placed by the mother in this pouch, 

 where it is hatched and undergoes its early development. The pouch 

 and the depression of the mammary area disappear when the breeding 

 season is over. In Ornithorhynchus there is no pouch and no depression 

 of the mammary area (for oviposition see p. 526). In the absence of 

 teats it would appear that the young take up the mammary secretion 

 by licking the mammary area. 



The ear is without a pinna. The males possess on the inner side of the 

 hind leg a horny spur, wliich is attached to an accessory ossicle on the 

 tibial side of the tarsus. Tliis spur is perforated, and transmits the duct 

 of a gland placed beneath the skin of the back of the thigh. The use of 

 tliis apparatus is not understood, but it is suspected that the gland secretes 

 a poisonous fluid and that the spvir is used in fighting, though this has 

 been denied. The young female possesses a small spur which however 

 cUsappears in the adult. 



In the skeleton the following points may be noticed in addition to those 

 already mentioned. The skull-ca\dty is large, and the cranial walls thin 

 and smooth. The sutm'es between the bones are largely obliterated. 

 The tympanic bone is annular and slender, and does not form a bulla. 

 The mallevis is large, but the incus is small ; the stapes is imperforate 

 and colmnelUform. The mandible is slender and with but slightly marked 

 coronoid process. The odontoid process long remains separate from the 

 axis, and the cervical ribs remain distinct for some time. In the scapula 

 the spine is placed on the anterior border and not in the middle of the 

 bone. In the peKdc girdle of Echidna the acetabulum is perforated. 

 The cerebr^im is a fair size, and is convoluted in Echidna, smooth in Orni- 

 thorhyyichus. There is a large anterior commissure, but no corpus cal- 

 losum. The cochlea* is only slightly bent, and alone amongst mammals 

 possesses a lagena with its papilla acustica. Echidna is edentulous, but 

 Ornithorhynchus possess ten or twelve multituberculate molar teeth, f 

 These are shed at a certain age (half -growth), being ejected and replaced 

 by horny teeth developed beneath them from the epidermal lining of the 

 mouth. The right auriculo-ventricular valve + is deficient on the septal 

 side, and there are no chordae tendineae, the papillary muscles being 

 attached to the edge of the valve (the left valve is also without chordae 

 tendineae). In Ornithorhynchus (not in Echidna) the muscular tissue 

 invades the membranous valve, and the muscular bands are inserted 

 directly into the margin of the opening, so that the valve is partly fleshy. 

 There is in Echidna an anterior abdominal vein § arising on tlie bladder 

 and distributed to the liver. 



The testes are in the abdomen, immediately behind the kidney, to 

 which they are suspended by a fold of peritoneum. There is a urino- 



* Pritchard, Phil. Trans., ISSl. 



t Poulton, Q.J. M.S., 29. Stewart, Q.J. M.S., 33, 1892, p. 229. 



i Lankester, P.Z.S., 1883, p. 8. 



§ Beddard, P.Z.S, 1884. 



