30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



width of the orbit, the type which represents the most extreme struc- 

 tural departure from normal mammals is realized among those 

 toothed cetaceans in which the maxillary bone passes obhquely up- 

 ward and backward from the orbit's anterior margin (pi. 6, figs, i 

 and 2 ; pi. 7, fig. 3 ; compare especially the skull of Kogia, pi. 7, fig. 3, 

 the genus in which these peculiarities are most obvious in the lateral 

 view, with that of a normal mammal, pi. 7, fig. i). Whatever may 

 have been its origin this second type of telescoping appears to be not 

 mechanically derivable from a condition in which the orbit had first 

 been roofed by two plates in horizontal position (compare Kogia, 

 pi. 7, fig. 3, with Stcnodelphis, pi. 7, fig. 2, and Hyperoodon, pi. 7, 

 fig. 4). Telescoping by this system carries the maxillaries upward 

 and backward from the anterior margin of the orbit at an angle of 

 from 50 to 70 degrees above the line representing the backward pro- 

 longation of the rostral axis and leaves the frontal broadly exposed in 

 the region lying above and behind the eye ; the orbit is immediately 

 roofed by a thick mass of the frontal alone, and not, as in the more 

 usual type of telescoping, by two thin approximately horizontal plates, 

 one formed by the frontal and the other by the maxillary. An ex- 

 planation of these conditions appears to demand the presence of 

 some factor radically different from those which brought about the 

 development of the typical dolphin type. Seemingly at a stage when 

 the maxillary had not yet been forced back over the orbit, the occipital, 

 and with it the entire braincase, may have been pressed forward suf- 

 ficiently to have modified the primitive form of the supraorbital region 

 of the frontal seen in the orbital roof of Pro"euglodon (pi. 5, fig. i) 

 and retained in ordinary dolphins (as in pi. 5, figs. 4 and 5) , by elevat- 

 ing it posteriorly so that it no longer lay in its original approximately 

 horizontal position. Thus when the maxillary, in its backward move- 

 ment, reached the mid-orbital level it would have been made to slope 

 upward away from the anterior edge of the orbital rim by a pre- 

 viously established upward slope of the frontal. While this hypothesis 

 is not based on any observed intermediate stages of structure, and 

 may therefore prove to be entirely wrong, it appears to ofifer an 

 explanation that involves fewer difficulties than those which are met 

 in any attempt to show that the conditions found in the sloping type 

 could have been developed after the maxillary had first established 

 itself over the orbit in a horizontal position. Another possibility 

 which must be considered is suggested by the structure which appears 

 to exist in Archccodelphis (pi. 5, fig. 3). In the only known specimen 

 of this animal the rostrum is broken away at its base, but the remain- 

 ing portion indicates that the entire rostrum was so depressed that its 



