290 ZAGLOSSUS. 
the lateral angle of the supraoccipital. There can be little doubt that these 
represent the parietals of other mammals, which through the extraordinary 
development of the median interparietal, have here become forced apart and 
occupy an inferior and lateral position. 
In the Echidna and especially in the Proechidna the parietal is nearly cov- 
ered underneath a large vertical expansion of the posterior end of the zygomatic 
arch. This (Plate 2, fig. 8, 7) is a flat scale-like bone, prolonged anteriorly into 
a pointed process that articulates with the dorsal surface of a long narrow exten-_ 
sion of the maxilla to form the zygomatic arch. Its posterior expansion is 
nearly semicircular in outline and dorsally may be in contact with the lateral 
edge of the interparietal. Ventrally it is extended slightly, at right angles to 
its lateral face, and lines the glenoid fossa for the articulation of the jaw. At 
its posterolateral edge it has an emargination where the external carotid enters, 
and passes forward to the orbit through a canal which thus runs between this 
scale-like expansion and the underlying bony wall of the cranium. This is 
the ‘temporal canal,’ considered a unique feature in mammals and doubtfully 
homologous “with a similarly placed canal in Anomodonts” (Gregory, 1910). 
This flat scale-like bone, perhaps as much from its shape as its position, is gen- 
erally considered a squamosal. In immature skulls it is easily removed expos- 
ing the underlying parietal and a second bone that occupies the wall of the 
cranium between the parietal and the lateral margin of the exoccipital, a bone 
‘mastoid.” If this flat scale-like bone be 
interpreted as a squamosal, it becomes necessary to conclude (1) that the jugal 
‘ 
considered by van Bemmelen the 
is quite wanting and (2) that the so-called “‘squamosal”’ does not form part of 
the brain-case, as can be shown in young specimens only. The first character 
is highly aberrant and the second apparently unique among mammals. Van 
Bemmelen endeavors to overcome the second difficulty by considering the bone 
underlying the supposed squamosal, as a greatly developed mastoid portion 
of the periotic. It seems, however, that another and simpler interpretation 
‘ 
is possible. Had van Bemmelen been able to flake off this ‘‘squamosal,”’ he 
would have found that the ‘‘mastoid” extended forward from the exoccipital 
to the posterior margin of the parietal. At its ventrolateral border it becomes 
hollowed underneath the articulating surface of the glenoid cavity formed by 
the extension of the ‘‘squamosal.’” The so-called ‘‘mastoid” bone, in short, 
seems to fulfil all the requirements of a true squamosal and fuses ventrally 
with a mastoid portion that lies on the ventral wall of the brain-case lateral to 
the basioccipital. 
