PROTOTHERIA (Neo-Lat. nom. pl., from Gk. rpiliros, pr5tos, first + therion, diminu tive of th)p, thy r, wild beast). The lesser and inferior of the two primary divisions of the Mannualia. It embraces only the small group represented at present by the Australian and Papuan egg-laying duckbill and echidnas, which constitute the order Monotremata (or Ornitho delphia) ; and possibly also the doubtful and little-known fossil group termed Allotheria or Multituberculata. The mammalian affinities of these extinct forms have been denied by some paleontologists, hut the weight of opinion views them as properly included in that category. Both externally and internally the monotremes show much that is distinctly mammalian, including the character of the brain. which in the echidnas at least is surprisingly large and well convoluted. The absence of a corpus callosum is the chief peculiarity differentiating it from the eutherian brain. The skeleton exhibits many archaic fea tures, one of the most striking of which is the presence of only the capitular head to the ribs. The shoulder girdle has other reptilian features, and a bone (the interelavicle) peculiar to the group. The digestive and circulatory systems
differ .little from the normal mammalian type, and the great distinction between the Prototheria and other mammals lies in the reproductive sys tem, and the fact that their eggs, instead of being minute and with little or no food-yolk, are large, contain much yolk, and therefore develop, so far as their early stages are concerned, after the meroblastic manner of a reptile's egg. The Pro totheria have a temporary ventral 'mammary' pouch in which the young are hatched, or to which they are transferred after hatching, and into which open the ducts of the mammary glands. This pouch is formed by a deep fold of the skin periodically develope'd in prepara tion for the young, and contains no teats, but nutrition is supplied by modified sweat glands. Its homologies are fully discussed by Beddard (Mantmalia, 1902), who refers to the work and writings of many investigators. See DCCKBILL ; ECHIDNA ; MAMMALIA,