NO. 5 TELESCOPING OF THE CETACEAN SKULL I9 



area which the nasals occupy, the frontals, owing to the slight for- 

 ward extension of the occipital shield and the exclusion of the 

 parietals from the middle of the vertex, share with Cetotherium and 

 its allies the maximum known degree of exposure on the vertex. 

 The surface of the relatively small occipital shield is noticeably tuber- 

 culate-roughened for muscle attachment. The rostrum and jaws have 

 been developed according to tendencies different from those seen in 

 other whalebone whales : rostrum moderately arched, rather narrow 

 at base, gradually tapering, straight-sided, and with a general inclina- 

 tion to deepening rather than to widening ; intermaxillaries con- 

 spicuously produced upward above maxillaries to form a raised rim 

 to the nasal cavity ; mandible very heavy, and so remarkably little 

 bowed outward that a straight line can be drawn from its base to its 

 tip without passing conspicuously outside of the general contour; 

 no definite coronoid process. The portions of the skull serving as 

 suspensorium to this unusually heavy mandible are smaller and less 

 specialized than in any of the other living baleen whales. 



The most extreme stage of the mysticete type of telescoping (pi. 8, 

 figs. 5, 6, /) is found in the finbacks and humpback. In all of these 

 w^hales, as in Balccna, Eubalcsna and Neohalmia, the occipital shield 

 is carried forward beyond the level attained by the anterior part 

 of the articular processes of the squamosal, a peculiarity which im- 

 mediately distinguishes them from the living RhachianectcS and the 

 extinct cetotheres. The general interlocking of the rostrum with 

 the cranium is in some respects like that which has been seen in 

 Rhachianectcs. But it has gone a definite step farther ; accompanying 

 the forward extension of the occipital shield the pointed antero- 

 external termination of the parietal, which projects slightly into the 

 frontal of Rliachiancctcs, has now been developed as a thin plate 

 extending far forward along the inner wall of the cavity above the 

 orbital wing of the frontal (pi. 6, fig. 3) in a direction parallel with 

 the nasal and nasal branches of the intermaxillary and maxillary, the 

 level of whose bases it overlaps. In the less pronounced examples 

 of this type (pi. 8, figs. 5 and 6) seen in Balcenoptera and Megaptera 

 a distinct strip of the frontal, lying between the forward-projecting 

 plate of the parietal and the backward-projecting nasal process of 

 the maxillary, connects the supraorbital wing with the narrow 

 exposure of the frontal on the vertex; but in the more pronounced 

 condition (pi. 8, fig. 7) present in Sibbaldus the frontal practically 

 disappears from the surface of this part of the skull, the parietal 

 almost applies itself to the outer margin of the nasal branch of the 



