292 ZAGLOSSUS. 



and in Ornithorhynchus forms the narrow bridge across the temporal canal. 

 What seems to be an analogous condition is. found in certain other groups, 

 as in the Cervddae, the Bovidae, and such Carnivora as Mungos ( = Herpestes) 

 in which the frontal process is largely developed at the posterior end and by 

 fusion with the postorbital process makes a complete bony orbit and separates 

 the temporal fossa posteriorly. The temporal canal in existing monotremes is 

 thus the temporal fossa greatly restricted by the scale-Uke expansion of the 

 posterior part of the jugal. In the Echidna this expansion is divided dorsally 

 by a rather deep notch into an anterior and a posterior lobe, the former of which 

 overlies the ventral edge of the parietal. In the Proechidna, the notch is much 

 less evident, the dorsal outline of the jugal more nearly hemispherical and the 

 anterior end extends far enough forward to overlap the posterior corner of 

 the frontal. 



Except for differeiices in relative size and fonn the bones of the palatal 

 and rostral regions are essentially similar in the Echidna and the Proechidna. 

 The termination of the palatals is slightly different, however, in the two. In 

 the former the medial portion of the palatal is produced posteriorly as a promi- 

 nent spine beyond the union with the pterygoid and there is a deep narrow 

 reentrant between the two palatals. In the Proechidna one specimen shows a 

 practically smiilar condition but in five other specimens the reentrant is broad 

 and shallow, and the palatals are rounded or truncated so as to merge with the 

 posterior outline of the pterygoids. 



The nasals in the Echidna do not extend posterior to the margin of the 

 orbit, whereas in the Proechidna they extend back to a point nearly opposite 

 the middle of the orbit. Since they overlap the frontals for nearly a centimeter 

 at this point, the latter bones appear for an absolutely shorter distance on the 

 dorsal aspect of the skull than in the Echidna. In both, the nasals taper distally 

 to a median point at some distance behind the nasal aperture, so that this latter 

 is bounded entirely by the premaxillaries which meet and unite dorsally. The 

 exclusion of the nasals from forming part of the boundary of the nasal aperture 

 is a singular and unique condition to which apparently no attention has hitherto 

 been paid. It is probably a specialized development, in part an accompani- 

 ment of the elongation of the rostrum. In the Echidna the distance from the 

 nasal aperture to the tip of the nasal bones is one fourth of the length of the 

 nasals. In the Proechidna it is slightly more than half their length, thus indi- 

 cating the relatively greater development of the rostrum m the latter animal. 

 In other mammals in which the rostrimi is greatly prolonged, for example the 



