NO. 5 TELESCOPING OF THE CETACEAN SKULL 1 3 



bones and the scapula have been alkided to by Winge in this con- 

 nection (Smithsonian Misc. Coll., Vol. 72, No. 8, p. 48, July 30, 

 1921). To account for the observed morphological conditions it now 

 appears to me unnecessary to assume that the three points of origin 

 were farther apart than adjacent families in some caniivore-like, 

 perhaps early creodont, stock ; one which might have also' given rise 

 to the modern carnivores and pinnipeds. When independently be- 

 ginning tO' undergo cetacean modifications one of these ancestral 

 forms may be supposed to have had some resemblance to Pterodon, 

 with a tendency toward narrowing the cranial portion of the skull 

 similar in character to the tendency which may now be seen in the 

 aquatic Cynogale (pi. 7, fig. i) among the Viverridce but carried to a 

 much greater extreme. It might have led to the zeuglodont type 

 (pi. 5, fig. i). The two others may have been more like Palceonictis 

 in general form of the skull, but with a tendency toward broadening 

 of the cranial region such as that which is now displayed by the 

 aquatic otters among the Mustelidce, thus leading to the modem 

 cetacean type. In these broader-headed animals two sets of peculiari- 

 ties, somewhat analogous on the one hand to those which I have de- 

 scribed as now existing in the fox and on the other to those now found 

 in the furseal and northern sea-lion, might be supposed to have sup- 

 plied the bases for developing, respectively, the skull of the toothed 

 cetaceans and that of the baleen whales. On account of their ap- 

 parently essential community of origin and fundamental structure 

 the three series should for the present be regarded as forming a 

 single order. All the known members of each series, including the 

 oldest fossils sufficiently well preserved to merit serious discussion, 

 have developed in strict accordance with the definite, and, as it would 

 seem, mutually exclusive tendencies of their respective groups. These 

 three groups show such entire independence throughout the geologi- 

 cal periods during which they are known to occur that, as is generally 

 recognized, they are best treated as distinct suborders. Their char- 

 acters may be summarized as follows : 



KEY TO THE SUBORDERS OF CETACE-\ 



Bones of both rostral and cranial portions of the skull retaining their 

 normal mammalian relationships ; braincase narrow and elongate ; teeth 

 present in about the maximum normal eutherian number, the hinder 



ones tending to disappear Arch.^oceti. 



[Cheekteeth retaining distinct traces of the inner portion of the crown. 



Protocctida;. 

 Cheekteeth without traces of inner portion of the crown 



Centra of vertebrae normal Dorndontid(e. 



Centra of vertebrse greath' elongated Basilosauridce.] 



