AND MAMMAEY FCETTTS OF THE ECHIDNA HYSTKIX. 
679 
But the mouth is proportionally wider, and has the form of a transverse slit (Plate XL. 
fig. 9, Plate XLI. fig. 4, n ) ; it is not circular. Upon the upper lip (ib. fig. 4, m), in the 
mid line between the two nostrils (a), is a small protuberance (e), corresponding to that 
in the young of the Ornithorhynchus paradoxus (ib. fig. 5, e), and wanting the cuticle. 
The tongue (ib. fig. 4, l) is broad and flat, extending to the “ rictus oris,” but very short 
in proportion to that of the parent, and of a very different shape. 
The traces of ears are less conspicuous than in the young Kangaroo, the conch being 
little if at all developed in the mature Echidna. The tail is much shorter than in the 
young Kangaroo, and shows as much proportional size as in the full-grown Echidna, in 
which it is a mere stump (Plate XXXIX. c) concealed by the quills and hair. 
The head is proportionally longer and more slender in the marsupial foetus of the 
Echidna than in that of the Kangaroo, and already, at this early period, foreshows the 
characteristic elongation and attenuation of that part in the mature animal. 
The form of the mouth as a transverse slit, in Echidna as in Ornithorhynchus , is a good 
monotrematous character of the young at that period, since in all true or teated marsu- 
pials the mouth of the mammary foetus has a peculiar circular and tubular shape. 
A scarcely visible linear cicatrix at the middle of the lower part of the abdomen is 
the sole trace of umbilicus (Plate XL. fig. 9). A bifid, obtuse rudiment of penis or 
clitoris (Plate XLI. fig. 3, d) projects from the fore part of the single urogenital or 
cloacal aperture, and in advance of the base of the tail-stump (ib. c). 
The brain, of which the largest part is the mesencephalon, chiefly consisting of a 
vesicular condition of the optic lobes, has collapsed, leaving a well-defined elliptical 
fossa of the integument indicative of the widely open “ fontanelle ” at the upper part of 
the cranium (Plate XL. fig. 10, Plate XLI. fig. 3, o ). The skin of the shrunk body 
shows folds indicative of the originally plump, well-filled abdomen. 
The fore limbs (Plate XL. figs. 11 & 12), in their shortness and breadth, foreshow 
the characteristics of those of the parent, which may be said, indeed, to retain in this 
respect the embryonic character with superinduced breadth and strength. The digits 
have already something of the adult proportions, the first or innermost of the five 
(fig. 12, i) being the shortest, the others retaining nearly equal length, but graduating 
shorter from the third to the fifth. The characteristic disposition of the digits is better 
marked in the hind limb (ib. figs. 13 & 14), the second (ii) already being the strongest 
and longest, the rest more rapidly shortening to the fifth ( v ) than in the fore leg ; the 
innermost (i), agreeably with the law of closer retention of type in the embryo, though 
the shortest of the five, is less disproportionately so than in the adult. 
It thus appears that the exterior characters of the young animal, figured in Plates 
XL. & XLI., accord with what might be expected, from the correspondingly immature 
characters in Macropus and Ornithorhynchus , in the offspring of the species alleged. 
In a question of this kind, as the liberal transmitters of the specimens were not them- 
selves the captors or original observers of the young with the mother, every possibility 
mdccclxv. 5 B 
