104 SKELETONS OF THE MONOTREMES, 



each has the appearance of Figure 6 of Plate XIX. They 

 take the place of the molar teeth, which, as Flower states, 

 upon either side rest upon the zygomatic process of the 

 maxilla, which is widened inferiorly into an oblong, concave, 

 roughened surface for their attachment. Owen claims that 

 Ornithorhynchns has no true malar bone. (P. 322.) 



Viewed superiorly, it will he seen that for the most part 

 the cranium of this nionctreme is smooth and flat, especially 

 the part anterior to the orbits. There is a conspicuous 

 foramen, on either side, piercing the nasal with a groove 

 leading from it to the front. Laterally, and opposite the 

 broad, thin, and transversely compressed zygoma, the side 

 of the cranium is marked by the temporal fossa ; it is shallow, 

 and of equal depth throughout its extent. The narrowest 

 part of the cranium is immediately anterior to this fossa. 

 In the post-basitemporal region there is a pair of large, 

 elliptical foramina, with another smaller pair between them 

 and the posterior nasal apertures. 



Between the molar teeth, the surface of the basis cranii 

 is smooth and concaved. On either side may be seen the 

 posterior palatine foramina (Fig. 9), which, next to the 

 interorbital diameter, is the narrowest part of the face. This 

 latter is much flattened, and from behind, forwards, becomes 

 gradually broader, to terminate distally as described in a 

 previous paragraph and here well shown in Figure 5 of Plate 

 XIX. "The infraorbital foramen," as Flower points out, "is 

 "very large, corresponding to the large size of the nerves 

 "distributed to the sensitive sides of the beak. The periotic 

 "has a wide and deep floccular fossa." 



The skull belonging to skeleton No. 2639 of the Army 

 Medical Museum has long been broken in two — a fracture 

 that now admits of a view of the interior of the brain case 

 through the absence of the entire anterior wall. 



With respect to the general form of the cranial casket, 

 the figures on the plates present more than can be gained 

 through any amount of description. In its interior there is 

 to be noted, however, the small olfactory fossa, pierced at itd 

 base by twin foramina, placed side by side transversely. 

 The anteriorly concaved wall rises behind this, the outer 

 angles of which exhibit well developed, posterior clinoid 

 processes. Falx cerebri are faintly pronounced and well ossi- 

 fied, especially the postero-median one, which is more or less 

 prominently produced. There appear to be considerable 

 differences in the outline of the foramen magnumi of the 

 Ornithorhynchus ; for in the smaller specimen of these two 



