MONOTREMATA. 
‘nections, and the proportions in which they 
enter into the formation of the skull, have been 
in great measure determined and described in 
that genus.* 
I have had the opportunity of investigating 
the composition of the cranium, a point so im- 
a in regard to the natural affinities of the 
onotremata, in the young Ornithorhynchi 
transmitted to the Zoological Society of Lon- 
don by Dr. Weatherhead ; and the comparison 
of this part of their anatomy has enabled me 
_ Fig. 169. 
369 
better to appreciate and understand peculiarities 
of the same part in the Echidna, the skull of 
which is here also described from original spe- 
cimens. 
In the cranium of a young but full-grown 
specimen of Echidna setosa, (g, fig.169, 170, 
171,) the four elements of the occipital bone are 
unanchylosed and are joined together by smooth 
linear harmonize. The basi-occipital (fig. 170, a) 
presents a six-sided rhomboidal figure, with the 
posterior margin notched to complete the lower 
Occipital and sphenoidal cranial vertebra, Echidna 
setosa. (Original. ) 
boundary of the large vertical occipital foramen, 
and thickened and smoothly rounded to form the 
inferior extremities of the two occipital con- 
dyles. These condyles are principally deve- 
loped from the ex-occipital elements (figs. 169, 
171, 6, b), which are expanded superiorly 
and terminate in an angle wedged in between 
the supra-occipital and petrous bones; they 
extend, but do not meet, above the occipital 
foramen, being separated by a notch closed by 
membrane in the recent state. The supra- 
occipital element (figs. 169 & 171, c) isa 
transversely oblong quadrilateral plate of bone; 
its short lateral margin is joined by a linear 
harmonia with the upper part of the os petro- 
sum, on each side; the wide anterior mar- 
gin is similarly articulated with the single 
parietal bone, and is slightly overlapped by its 
posterior margin; this representative of the 
deltoid suture runs straight across the posterior 
and upper part of the skull. 
Fig. 171. 
Occipital region of skull, Echidna setosa. ( Original. ) 
In the specimen in which the preceding 
condition of the occipital vertebra was mani- 
fested there was no trace of sagittal suture ; 
the upper and middle region of the cra- 
nium was covered by a single broad, slightly 
convex, parietal bone, (fig. 169, d,) joined 
posteriorly, as above described, with the supra- 
occipital, laterally with the petrous and sphe~ 
noid bones, and anteriorly with the sphenoid 
* See Legons d’Anatomie Comparée, Ed. 1837. and frontal bones, which the agin overlaps 
B 
VOL. III. 
