130 METATHERIA 
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 vetebre 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 vene cave,' 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 feces, 
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 
mamme 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 Petawrus (Belideus) breviceps (Forbes, Proc. Zool. Soc. 1881, 
p. 188). 
