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, j) 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 

 considered by van Bemmelen the "mastoid." If this flat scale-like bone be 

 interpreted as a squamosal, it becomes necessary to conclude (1) that the jugal 

 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 

 underljdng 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 aU 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. 



