4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



doubtful that the known zeuglodonts were ancestral to any of the 

 recent whales it seems probable that they came from a terrestrial stock 

 which was nearly related to the latter's forerunners ; and there can 

 be little question that in certain details of structure they possess 

 features which are morphologically intermediate between those of 

 early land mammals and those of some living cetaceans. These 

 features will be dealt with in discussing the probable course of de- 

 velopment of the modern whales (pp. 11-12). 



GENERAL CONDITIONS IN MODERN CETACEA; SUGGESTIONS 



AS TO THE POSSIBLE MECHANICAL ORIGIN 



OF THESE CONDITIONS 



Concerning the modern cetacea the most conspicuous facts are these : 

 (a) That the telescoping of the skull was far advanced in the earliest 

 known extinct genera, and (b) that this process has developed ac- 

 cording to two different plans. No transitional stages between these 

 two plans of development are known ; and no fossil cetacean has yet 

 been described in which the skull has been definitely shown to be so 

 constructed as to furnish the elements needed for the development 

 of both. While there are important variations in details, the funda- 

 mental schemes of the two plans or types are as follows. In one type 

 (pi. I, figs, la, lb) the entire proximal portion of the maxillary 

 (a. pr.) passes up over the frontal and backward to approach or meet 

 the supraoccipital at a level behind the orbit {orb.) ; laterally it 

 spreads out over the expanded supraorbital wing of the frontal. 

 Backward motion of anterior elements is the most obvious feature of 

 this first process. In the other type (pi. i, figs. 2a, 2b) the broad 

 outer part of the hinder maxillary border (o. pi.) projects obliquely 

 downward and backward under the anterior margin of the great 

 supraorbital wing of the frontal, while the narrow inner part {a. pr.) 

 fits closely into the body of the frontal on the upper surface of the 

 forehead ; the upper surface of the expanded supraorbital wing of 

 the frontal is thus left bare. As though further backward progress 

 of the maxillary were rendered difficult by this double interlocking 

 of maxillary and frontal, telescoping is chiefly accomplished by for- 

 ward extension of the occipital and parietal to and beyond the median 

 orbital level (orb.). Forward motion of posterior elements is the 

 most obvious feature of this second process. The first plan is peculiar 

 to the dolphins and toothed whales, the second is confined to the 

 baleen whales. 



The opposite trends of motion in the elements chiefly concerned 

 with the telescoping process according to the two plans are especially 



