illary bone (fig. 169, m_) extends backwards as 
far as the posterior boundary of the zygomatic 
_ ortemporal fossa; the palatal process extends 
along the floor of the orbit in a similar form 
and to nearly the same extent. The orbit is 
marked off from the temporal fossa by merely 
a slight ridge extending down and across the 
suture joining the frontal and sphenoid bones. 
The skull of the Echidna differs from that of 
the edentulous Manis and Myrmecophaga in 
_ the completion of the zygomatic arches, in the 
unclosed state of the tympanic cavity, in the 
_ large size of the foramen incisivum, and the 
“surrounding of the external nostrils by the 
rmaxillary bones alone: it differs also in 
smaller relative distance between the poste- 
or palatal fissure and the superior maxillary 
bones, and in the apparent absence of the pala- 
tine bones, the presence and interposition of 
_ which between the pterygoid and maxillary 
atal plates elongates the palate in the pla- 
al Anteaters at the part where it is rela- 
tively shorter in the Echidna. In the modi- 
_ fication of the pterygoid plates of the sphenoid 
_ to complete the posterior nasal canal, the 
_ Echidna manifests an interesting resemblance 
with the great Anteater; but it differs from 
this, as from every other mammiferous species, 
_ in the palatal plates contributed by the petrous 
bones to the broad posterior part of the roof of 
the mouth which supports the horny palatal 
h. Cuvier describes the posterior palatal 
re as extending between the palatine bones, 
__and therefore regards the plates, which are here 
affirmed to be developed from the petrous bone, 
____asbeing the pterygoid processes of the sphenoid ; 
and, according to this view, he truly observes 
_ that their horizontal position is very remark- 
e;* but he might have added, that their 
e in the formation of the tympanic cavity 
‘was not less so. The same determination of 
ie ie bones composing the posterior part of the 
osseous palate of the Echidna is repeated in 
_ the Lecons d’Anatomie Comparée, 1837, 
43 454. If, however, the sphenoid be sepa- 
_ tated from the occipital bone, which was easily 
_ done in the young skull of the Echidna repre- 
_ sented in figs. 169 and 170, the horizontal plates, 
described by Cuvier as pterygoids, are left 
behind, not as separate bones, but as conti- 
_ huous portions of the petrous elements of the 
__ temporal, which form, at the same time, part of 
_ the base of the cranial cavity, complete the 
_ inner wall of the tympanum, and the anterior 
part of the Eustachian groove. The palatines 
of Cuvier are developed from the sides of the 
_ basi-sphenoid, and almost immediately bend 
_ inwards and meet below the nasal canal, which 
_ they thus prolong posteriorly, as in the Myr- 
_ mecophaga; and they are separated poste- 
_ fiorly, also, as in that genus, by an acute fissure, 
_ Presenting unequivocally the same modifications 
‘ 
? 
x 
4 
_* Cuvier says, ‘‘ Une echancrure digne et pro- 
_fonde sépare les palatins en arriére. Le plan de 
_ chacun d’eux est continué en dessous par une apo- 
physe ptérygoide, qui ici, chose bien remarquable, 
_ €st horizontale ou a peu pres: elle contribue a 
____ former la cavité de la caisse.”’--Ossem. Fossiles, 
_ Le.p.146. Meckel is silent on this subject. 
MONOTREMATA. 
371 
which characterize the pterygoids in the pla- 
cental Anteaters, and in the Crocodile. The 
suture dividing the pterygoids from the pala- 
tines in the Echidna is obliterated, if it ever 
existed ; or the true palatines may be confluent 
with the palatine processes of the maxillary 
bones. 
Some of the Marsupials, as the Wombat,- 
resemble the Echidna in the open state of the 
tympanic cavity in the dry skull; but the most 
essential points of correspondence with the 
cranial anatomy of the Echidna are found, as 
might be expected, in the Ornithorhynchus. In 
this Monotreme the tympanic cavity (fig. 173, 
A, k) is a simple excavation at the under part of 
the petrous bone; the periphery of the opening, 
which looks almost directly downwards, is en- 
compassed by the tympanic and malleal bones, 
(fig-173, D, a, b,) the outer and anterior part of 
the circle being formed by the os tympanicum. 
The tympanic cavity is relatively smaller, but 
is defended posteriorly by a larger process sent 
downwards by the petrous bone near its outer 
side. The petrous bone here forms no palatal 
process, but the bony roof of the mouth is ter- 
minated by the pterygoid plates, (fig. 173, a, 
t, ) which meet below the nasal canal, as in the 
Echidna, but are not divided by any posterior 
fissure. Ina skull of the Ornithorhynchus, in 
which the suture dividing the palatine processes 
of the maxillary bones from the bony palate 
posterior to them remains, there is no trace of a 
division between the pterygoid and palatine 
bones, which contribute to complete the osseous 
palate (fig. 173, a, e). 
Lhe occipital bone of 
the young Ornithorhyn- 
chus corresponds with 
that of the Echidna in the 
relative size and position 
of its four component 
parts. The ex-occipitals 
are shown at fig. 172, 
b, 6, and the supra-occi- 
pital at c. The petrous 
element of the temporal 
(e) likewise sends a thin 
plate to form the poste- 
rior part of the side of the 
cranium, but it does not 
intervene between the pa- 
rietal bone and squamous 
part of the temporal, as 
in the Echidna. The 
middle of the upper mar- 
gin of the cranial plate Skull of young Ornitho- 
of the petrous bone is "hynchus. ( Original. ) 
notched, and a small vacuity here intervenes 
between the petrous and parietal bones, which is 
closed by the squamo-temporal (f), the upper 
margin of which overlaps the descending pos- 
terior angle of the parietal bone (d). The form 
of the squamous element of the temporal is 
very remarkable in the Ornithorhynchus: its 
ascending cranial or proper squamous portion 
is asmall sub-quadrilateral bony scale, narrower 
antero-posteriorly, but higher than the corres- 
ponding part in the Echidna; articulated by 
its squamous margin with the parietal and 
2B 2 
Fig. 172. 
