The Xeiiartlira comprise t h e e families, the Kradypodidae, or sloths, tlie Dasypoclidae, or ai*madillos, arid the Myrmecophagithe, or aut-eaters. The study of reproduction has been neglected in these groups, because of the difficulties of obtaiiiiiig siificicrit material. Howc.er, in recent pears, Feriiaiidez ('09) aiitl Newman aiid Patteimii ('10) have 01)taiiiecl suficieiit armadillos to malie important advaiices in 0111. k1iowlc~dge of the devclopmcnt a i d the eayly stages of placentation in tliat geuiis, and Wislocaki ( '2-i) has studied placeiitntiori in a series of sloths. Our kiiowletlge of reprotlnctioii iii the aiit-eaters, on tlie other Iiaiicl, still clcpeuds upoii tlie stiiiij7 of isolatctl specimeiis, ant1 even these a r e few in iiumlier. Nevertheless, tlic stud)-of these isolated cases is importaitt, if n.e arr t o make any progress iii the widerstandiiig of the mutual affinities of tlic giwnps n-itliiii the order of the X'eiiarthra.
The literature 011 tlie plnceiitatioii of aiit-caters coiitaiiis oiily a few brief references wliicli seldom acqiiaiiit us with more tliaii a barcl statement :IS to the size ant1 shape of the placenta. illeyer, in 1839, esamiiietl ;I gravid uterlls of ,\l~~erriecopliaga clidactyla mid t3cscr.il)ed a single, Tvelldeveloped fetns witliiii the uterus with the liead prescntiiig. r i i h e fetus possessed a coat of long hair. Velcker, in 1861, mentioiiecl that in i\lyrmecopliaga tctradactyla tht. placenta is fnngiform, while Millie-Etlwartls, in 1872, statctl that the