144 Dr. W. T. Caiman on the the hairs slaty, with pale buffy tips. Crown of head like back. Dark orbital rings broad, strongly marked, extending forwards on to the sides of the muzzle. Cheeks and chin clearer bnffy. Outer sides of arms and legs like back, inner sides like belly ; hands and feet practically naked, pale brownish. Tail furry at its base for a shorter distance than usual, the fur, which is coloured like that of the back, ex-tending for only about an inch and being surpassed posteriorly by the outstretched feet ; remainder of tail naked, as usual, but instead of being white terminally it is uniformly pale brown to the end, at least above, the under surface being in one specimen slightly paler terminally. Skull with well-expanded zygomata and broad interorbital region, with overhanging postorbital ledges. Teeth large, of the usual proportions in this group. Dimensions of the type (measured in skin) : — Head and body 187 mm. ; tail 245; hind foot (s. u.) 23; ear 19. Skull : basal length 39*5 ; greatest breadth 25 ; nasals 18*5 X 6*2 ; interorbital breadth 7*6 ; breadth across post-orbital processes 9*4; breadth of brain-case 15; palate length 23*5 ; combined length of three anterior molariform teeth 7'7. Hob. Sarayacu, Oriente of Ecuador. Type. Female (young adult). B.M. no. 80. 5. 6. 77. Collected by Mr. Clarence Buckley. An old male also in collection. This opossum shares with M. regina * alone of the present group the distinction of having a wholly brown tail, not turning to white at its end. From that species it is separated by its duller and less yellowish belly-colour, broader skull, and larger molars. XVIII. — On the Classification of the Crustacea Malacostruca. By W. T. Calm an, D.Sc. In the course of preparing a general account of the Crustacea for a forthcoming volume of Prof. E. Kay Lankester's 1 Treatise on Zoology ' I have been led to discard the com-monly accepted classification of the Malacostraca and to adopt a scheme which was briefly outlined by Dr. H. J. Hansen some ten years ago. The object of the present * Ann. & Mag. Nat. Hist. (7) ii. p. 275 (1898).