26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



supraorbital process of the frontal; but its position in the region 

 directly over the middle of the orbit is always (see particularly the 

 constancy of this feature in two such diverse types as Stenodelphis 

 and Hypcroodon, pi. 7, figs. 2, 4), like that of the dorsal surface of 

 the expanded process, essentially horizontal in striking contrast to 

 its oblique position in Physetcr, Plafanista (pi. 6, figs, i, 2) and Kogia 

 (pi. 7, fig. 3). The orbital cavity, in other words, is roofed over, at 

 least in its median portion, by two flattened plates of bone which lie 

 in or nearly parallel to a plane representing the backward prolonga- 

 tion of the median horizontal plane of the rostrum. With the crush- 

 ing together of the anterior and posterior elements of the skull the 

 narial passages have been forced back so that they occupy an almost 

 vertical position closely following the contour of the convex outer side 

 of the anterior wall of the braincase, down which they extend like 

 a pair of gutter pipes. The nasals and the median portion of the 

 frontals (pi. i, fig. la) have been pushed backward until they no 

 longer roof the proximal ethmoid region. The palatine has a much 

 reduced exposure on the roof of the mouth (pi. i, fig. ib) ; but it has 

 developed a large new narial plate which forms an important part 

 of the anterior wall of the narial passage. 



This stage of telescoping is found in the great majority of members 

 of the suborder. It was fully established in the Miocene of both 

 Europe and America ; and its fundamental importance seems clearly 

 indicated by the fact that it has remained constant in animals whose 

 skulls and dentitions have diverged in other characters as strikingly 

 as those of Sqiialodon and Monodon, Eurhinodelphis and Orcella, or 

 Hyperoodon and Orcinus. That it should present numerous 

 secondary degrees and variations of development would, however, be 

 expected. The conditions which in some respects are the least far 

 removed from the ordinary mammalian type and from the Agoro- 

 phiidcB are most clearly shown by the extinct Sqiialodon and the exist- 

 ing genera Inia and Lipotes. Here the braincase, though relatively 

 larger than in Agorophius and Ar dice del phis, has not reached its 

 maximum development, and the region of juncture between the 

 occipital and frontal bones has not pushed forward into any noticeable 

 proximity to the level of the orbit ; a considerable part of the frontal 

 remains visible on the side of the braincase ; the temporal fossa has 

 not sufifered marked reduction (its size, when the skull is viewed from 

 behind, obviously exceeds the combined area of the occipital condyles 

 and the foramen magnum). A slight variation of this condition 

 occurs in Stenodelphis (pi. 7, fig. 2) : frontal conspicuously exposed 



