NO. 5 TELESCOPING OF THE CETACEAN SKULL 1 5 



According to one of these tendencies the forward movement of the 

 posterior elements of the skull seems to have held precedence over 

 the backward movement of the anterior elements ; according to the 

 other the opposite seems tO' have been true. While, within each main 

 type, the predominance of one movement is usually evident, there are 

 phases of each type in which the two movements are so combined as 

 to give rise to a somewhat balanced or less one-sided form of tele- 

 scoping, and others in which the skull appears as if it may have been 

 first subjected to one movement and later to the other. In a general 

 way these varying conditions are distributed somewhat as follows : 

 occipital thrust conspicuously dominant in the balasnoids and less so 

 in the balaenopteroids ; maxillary thrust conspicuously dominant in 

 most of the odontocetes ; the two thrusts more intimately combined in 

 the odontocetes of the physeterine type and Platanista; a final occipital 

 thrust subsequent to a strongly developed maxillary thrust in the 

 ziphiids. In the following discussion of the details of telescoping 

 among the cetaceans whose skulls are sufficiently known to show these 

 essential characters the use of expressions conveying the ideas of 

 early and late or before and after, is (unless something else is clearly 

 shown by the context) to be understood as applying to the process 

 and not to time; a very late stage of a process might be quickly 

 reached in one animal at a very early geological time, while an early 

 stage might persist in another animal, or in a dififerent part of the 

 same one, to the present day. 



DETAILS IN THE BALEEN WHALES 



In the general structure of their skull the baleen whales have 

 departed less widely than the toothed cetacea from the ordinary 

 mammalian type. The choanje still lie distinctly behind the anterior 

 nares as in other mammals. The bones forming the nasal passage 

 retain a general arrangement which is essentially normal, agreeing 

 with the conditions present in the furseal and sea-lion in their funda- 

 mental relationships, including the complete roofing over of the 

 proximal ethmoid region by the nasals and the median portion of the 

 frontals, and the exclusion of the palatine from the anterior wall of 

 the narial passage ; primitive features which have for the most part 

 disappeared in all known adontocetes except the agorophiids and 

 physeteroids, where some or all of them may persist. The maxillary 

 retains the orbital portion of the body of the bone, while the rostrum 

 is never developed into the attenuated beak which is characteristic of 

 many though not all toothed cetaceans. 



