(84) 
reality it is separated from this patch longitudinally by a suture that 
runs through the lateral part of the tympanic cavity. By its form 
and position this patch resembles the mastoidal part of the temporal 
of other Mammals, but as for its size and independence, it may 
be compared to the opisthoticum and epioticum, taken conjointly, of 
Sauropsida. 
This mastoidal part of the skull-wall is a chondrostosis, which 
fact is in itself sufficient to forbid its comparison with a squamosal, 
a comparison one might otherwise be much inclined to make, consi- 
dering that the dermal bone which is situated on its outer surface might 
easily be mistaken for the jugal, with which it shows many points 
of resemblance, and which, but for this hypothesis, must be con- 
sidered as absent in the Monotreme-skull. 
IV. Arcus zygomaticus. 
The malar arch of Monotremes is made up of two bony processes, 
running side by side for the greater part of their length. The 
anterior belongs to the maxillary, the posterior to the above-men- 
tioned dermal bone, that I take to be the squamosal. A jugal bone 
is totally absent in E. In O. on the contrary a little prominence 
occurs on the dorsal side of the arch, marking the limit between 
orbital and temporal fossae. In some skulls this prominence was 
found separated by a suture from the underlying zygomatic process 
of the maxillary. Most probably we may look upon it as the last 
remnant of a disappearing jugal. The foetal O. skull did not show 
any trace of it. 
V. Canalis temporalis. 
Between the squamosal and the wall of the primordial-cranium 
(mastoidal bone), a canal is left open from before backwards in 
both E. and O. 
In no other Mammalian skull a trace of such a canal was found. 
In E. it is longer but narrower, in O, shorter but wider. Its lumen 
is filled up with fibres of the temporal muscle. 
Moreover in E. an artery penetrates into the skull-wall by the 
posterior opening of this canal, but immediately leaves it to con- 
tinue its way through the diploé of mastoidal, parietal and frontal 
as far as the ethmoïdal region. Hyrrr calls it art. occipitalis. 
1 
