NO. 5 TELESCOPING OF THE CETACEAN SKULL 5 



well shown by figures 2 (Plataiiista) and 3 (Balccnoptera) of plate 6. 

 Here it will at once be seen that in the toothed cetacean the long 

 axes of the frontal, parietal and squamous portion of the squamosal 

 slope upward and backward as though forced into this position by 

 some backward-crushing power acting through the elongated ascend- 

 ing process of the maxillary. In the baleen whale, on the contrary, 

 these axes all slope upward and forward as though they had been 

 dominated by a f oreward-crushing pow' er acting through the elongated 

 occipital shield. The horizontal position of the same axes in a 

 normal skull, where no elongation of the maxillary or the occipital has 

 taken place, is shown in figure i of plate 7. 



The skull of a finback w4iale is thus seen to be telescoped in a 

 manner so unlike that of a dolphin that it is at first difficult to under- 

 stand how two such opposite types could have originated. The 

 fossils, thus far described give no clue to the probable history of the 

 two processes of telescoping. Such extinct genera as Cetotherium, 

 ArclK€odclphis, and Agorophius show conditions less advanced than 

 those found in living forms ; but no extinct cetacean has yet been 

 made known in which there is certainly a confusing or blending 

 of the two types, or in which there has been demonstrated the presence 

 of a structure from which both plans could be elaborated. 



A clear understanding of some of the- more important mechanical 

 features of the tv\^o types can be gained by examining the different 

 ways in which contact betw^een the bones concerned in the telescoping 

 process is established in other mammals. Without attempting to 

 work the subject out in detail I have found that in mammals which 

 have a broad area of contact between the maxillary and frontal above 

 the anterior part of the orbital rim two kinds of relationship can be 

 seen : (a) The maxillary may slide over the frontal in the form of 

 a thin plate or tongue of bone (fox, pi. 2, fig. i, a. pr.), or (b) the 

 edges of the bones may be solidly locked by interdigitating processes 

 (bear) or by slipping the edge of the maxillary obliquely downward 

 into the substance of the frontal (furseal, sea-lion, pi. 2. fig. 2, a. pr.). 

 Sometimes the two methods are combined so that the maxillary 

 extends freely up over the frontal alongside of the nasal but sends 

 a well developed flange downward into the frontal at the edge of the 

 orbit (cat, some mustelines). Turning to the line of contact between 

 the occipital and parietal it is again seen that various kinds of juncture 

 occur: the bones may come solidly together (bear, raccoon), or either 

 may slightly override the other (parietal over occipital in fox, pi. 2, 

 fig. I, and domestic cat, occipital over parietal in furseal and sea- 



