2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



been produced such specialized types of skull as are seen in elephant, 

 seacow, brontothere, anteater and man. In striking contrast to this 

 kind of remodeling, the process which the skulls of all known 

 cetaceans except the zeuglodonts have undergone is a highly developed 

 system of " telescoping " ; that is, the portion of the skull lying behind 

 the rostrum has been shortened, not so much by a reduction of the 

 anteroposterior diameter of individual bones (except the parietal), 

 as by a slipping of one bone over another or by the interdigitating 

 of some of the elements. Alteration of contact-relationship is here 

 not the exception but the rule. In this manner unusual conditions 

 have arisen ; such as contact of the premaxillary and supraoccipital 

 (pi. 7, fig. 3; pi. 8, fig. 7), the presence at one transverse plane of 

 parts of the nasal, premaxillary, maxillary, parietal, and frontal (pi. 8, 

 figs. 6, 7), the partial covering of the supraorbital process by the 

 lacrimal (pi. 5, fig. 6), the entire covering of the palatine and ali- 

 sphenoid by the pterygoid (p. 31) or the presence at one perpendicular 

 plane of parts of the occipital, frontal and nasal (pi. 8, fig. 9). .These 

 rearrangements of the elements of the skull are not in any general 

 sense mere degenerative changes. They afl:'ect the skull's funda- 

 mental structure, and they are peculiar to cetaceans. With little 

 doubt, therefore, they represent responses to stimuli which are in 

 some way directly connected with the conditions under which these 

 animals live — perhaps most particularly with the habit of rapid, 

 fish-like, pelagic swimming ; in other words they are active adapta- 

 tions in one of the parts of the skeleton most essential to cetacean 

 existence. Hence the varying degree of their perfection may prop- 

 erly be regarded as indicating the varying extent to which different 

 cetaceans have departed from the original land mammal type. While 

 the fact of telescoping in cetacean skulls has long been known, the 

 details of the process throughout the group have never to my knowl- 

 edge been studied, nor does any one appear to have attempted to 

 show what phylogenetic importance these details may present. 



Considered purely from this point of view the cetacea are divisible 

 into two groups : those in which telescoping is entirely absent and 

 those in which it is conspicuously developed. There are no known 

 intermediate stages between these two conditions. All of the zeu- 

 glodonts belong in the first group. All of the other cetaceans whose 

 skulls have been described belong in the second. As the zeuglodonts 

 are not known to have existed since early Tertiary times, the members 

 of the second group, abundantly represented in the seas of to-day, 

 may be alluded to collectively, whether living or fossil, as modern 

 cetacea. 



