NO. 5 TELESCOPING OF THE CETACEAN SKULL 7 



that the overlapping of the posterior elements is strictly of the sea- 

 lion type, and that the specialized structure of Spalax is based on the 

 working out of a condition latent in alHed rodents whose skulls have 

 remained normal. 



While the obvious superficial characters of the two methods of 

 telescoping as just described are easily observed in adult cetacean 

 skulls, the more essential underlying features of the processes can 

 only be studied in specimens young enough to permit of disarticula- 

 tion.^ When such material is examined it becomes evident that the 

 key to an understanding of the differences is probably to be found 

 in the structure of the posterior portion of the maxillary, the region 

 whose morphology appears to be more fundamentally essential in this 

 connection than that of any other part of the skull. In the baleen 

 whales the orbital portion of the body of the maxillary^ is present 

 and well developed as a large " horizontal ventral " plate projecting 

 conspicuously behind and beneath the infraorbital foramen. The 

 jugal comes in contact with the postero-external portion of this 

 plate, at a level below, behind and considerably lateral to the foramen 

 (pi. I, fig. 2&; pi. 3, fig. 3 ; pi. 4, fig. 3) . In the toothed cetacea (pi. i, 

 fig. ih; pi. 3, figs. 5 and 6; pi. 4, fig. 2) the horizontal orbital portion 

 of the maxillary is absent, and the jugal is interlocked with the 

 maxillary at a point close to the infraorbital foramen and usually in 

 front of it ; always at a level very different from that at which contact 

 is established in the baleen whales. It is therefore evident that the 

 difference between the two kinds of relationship of the maxillary to 

 the frontal in the modern cetacea is much greater and more significant 

 than is implied by the usual idea that in the toothed whales the 

 maxillary passes backward over the frontal while in the baleen 

 whales it passes backward mostly under the frontal. For the large 

 and conspicuous part of the maxillary which passes under the frontal 

 in the latter group is a structure which the toothed cetacea do not 

 possess (compare especially pi. i, figs, ih and 2h; pi. 3, figs. 3 and 6; 

 pi. 4, figs. 2 and 3). On the other hand the part of the maxillary 

 which is common to the two groups, the ascending process, has in 

 reality a homologous relationship to other structures in both types ; 

 the essential part of the great apparent difference is merely that in 



^ I have at my disposal young representatives of the genera Balceuoptera, 

 Berardius, Delphinaptcriis, Delphinus, Globiccphala, Grampus, Kogia, Lageno- 

 rhynchns, Physeter. 



^ In normal mammals this part of the maxillary forms the floor of the 

 anterior region of the orbit (see Jayne, Mammalian Anatomy, pt. i, fig. 266). 



