6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



lion, pi. 2, fig. 2). It is not now essential to try to explain the 

 significance of these various forms of interlocking in mammals whose 

 skulls are not telescoped. The important point is to recognize that 

 among such mammals are found some primary structural elements 

 from which it would appear to be mechanically possible to initiate 

 both of the processes that have been elaborated in the skulls of the 

 modern whales. In the fox (pi. 2, fig. i) may be observed the 

 combination of peculiarities which, so far as they go, are the ones 

 seemingly needed to lead to the toothed whale type. Anteriorly the 

 broad ascending process of the maxillary (a. pr.) overrides the 

 margin of the frontal in a direction which, if continued, would carry 

 it freely back over the base of the postorbital process and beyond 

 to the highest point of the braincase. At the back of the skull the 

 occipital fits so solidly against the bones in front of it that any for- 

 ward progression of the upper part of the occiput would apparently 

 need to be accomplished by eating intO' the substance of the hinder 

 portion of the parietals, a process which can easily be imagined to 

 present greater mechanical difficulty than the unobstructed backward 

 sliding of the maxillaries over the upper surface of the frontals. 

 The final approximation or contact of occipital and maxillary might 

 therefore be expected to take place at a level decidedly behind the 

 orbit, exactly as happens in the great majority of toothed cetacea 

 (see especially pi. i, fig. la; pi. 5, fig. 5; pi. 7, figs. 2, 4). In the 

 furseal and northern sea-lion (pi. 2, fig. 2), on the other hand, a com- 

 bination occurs which would seem to furnish a structural beginning 

 that might lead equally well toward telescoping according to the other 

 plan. Here the maxillary (a. pr.) is so firmly locked with the frontal 

 as to have the appearance of opposing a serious check to backward 

 movement, while the occipital overlaps the parietal as freely as the 

 maxillary overlaps the frontal in the fox. Thus the mechanical ele- 

 ments are provided which might finally lead to the forward extension 

 of the occipital shield to the level in front of the orbit which it 

 reaches in the baleen whales. No series of ancestral or of less 

 -Specialized living forms is known in which the skulls show stages in- 

 termediate between the conditions seen in the brain case of the sea-lion 

 and the baleen whales, but a nearly parallel morphological series 

 can be observed leading from the large occipital shield which con- 

 spicuously overrides the parietals in Spalax (pi. 8, fig. 8) back through 

 such cricetine rodents as Myospalax and the more f ossorial species of 

 Arvicola to a completely unmodified occipital region like that of 

 Neotoma. Longitudinal sections of the skulls of these rodents show 



