i *> ME TA THERIA 



of the other teeth have cut the gum, and therefore quite function- 

 less. It must further be noted that there are some Marsupials, 

 as the Wombat, Myrmecobius, and the Dasyures, in which no such 

 milk-tooth, even in a rudimentary state, has yet been discovered, 

 possibly in some cases from want of materials for observation at 

 the right stage of development. 



Epipubic or marsupial bones are present in both sexes of nearly 

 all species. In one genus alone, Thylacinus, they are not ossified. 

 The number of dorso-lumbar vetebra? is always nineteen, although 

 there are some apparent exceptions caused by the last lumbar being 

 modified into a sacral vertebra. The number of pairs of ribs is 

 nearly always thirteen. The tympanic bone remains permanently 

 distinct. The carotid canal perforates the basisphenoid. The 

 lachrymal foramen is situated upon or external to the anterior margin 

 of the orbit, and there are generally large vacuities in the bony 

 palate. The angle of the mandible is (except in Tarsipes) more or 

 less inflected. The hyoid bones have always a peculiar form, 

 consisting of a small, more or less lozenge -shaped basi-hyal, broad 

 cerato-hyals, with the remainder of the anterior arch usually 

 unossified, and stout, somewhat compressed thyro-hyals. There are 

 two anterior venae cava?, 1 into each of which a "vena azygos " 

 enters. In the male the testes are always contained in a scrotum, 

 which is suspended by a narrow pedicle to the abdomen in front of 

 the penis. The vasa deferentia open into a complete and continuous 

 urethra, which is also the passage by which the urine escapes from 

 the bladder, and is perfectly distinct from the passage for the feeces, 

 although the anus and the termination of the urethro-sexual canal 

 are embraced by the same sphincter muscle. The glans is often 

 bifurcated anteriorly. In the female the oviducts never unite to 

 form a common cavity or uterus, but open separately into the 

 vagina, which at least for part of its course is double. The 

 mammae vary much in number, but are always abdominal in 

 position, having long teats, and in most of the species are more 

 or less enclosed in a fold of the integument forming a pouch 

 or marsupium, though in some this is entirely wanting, and the 

 newly -born, blind, naked, and helpless young, attached by their 

 mouths to the teat, are merely concealed and protected by the 

 hairy covering of the mother's abdomen. In this stage of their 

 existence they are fed by milk injected into their stomach by the 

 contraction of the muscles covering the mammary gland, the 

 respiratory organs being modified temporarily, much as they are 

 permanently in the Cetacea — the elongated upper part of the 

 larynx projecting into the posterior nares, and so maintaining a free 

 communication between the lungs and the external surface 



1 Except in Petaurus (Belidcus) brcviceps (Forbes, Proc. Zool. Soc. 1881, 

 p. 188). 



