NO. 5 TELESCOPING OF THE CETACEAN SKULL 2^ 



del phis might be regarded as giving some hint of an early stage in 

 the development of conditions like those now seen in Platanista (pi. 6, 

 fig. 2) and the physeterines (pi. 6, fig. i ; pi. 7, fig. 3). In the genus 

 Xcnorophns, however, specialization appears to have advanced in 

 directions which have not been followed by later members of the 

 group. This is shown by the wide spreading of the lacrimal over 

 the supraorbital process of the frontal to the level of the hinder 

 margin of the eye ; by the abrupt widening of the intermaxillary in 

 the region behind the narial aperture, this widened portion forming a 

 thin plate spreading outward underneath the maxillary over appar- 

 ently the basal half of the supraorbital process ; and finally by the 

 very abrupt and conspicuous depression of the maxillary in the 

 region immediately in front of the orbit, the sudden slope thus 

 formed giving the horizontal portions of the bone lying respectively 

 above the orbit and above the roots of the teeth (in the regions 

 marked «7.r and a. pr, pi. 5, fig. 6) somewhat the profile of upper and 

 lower river terraces separated by an escarpment. The under-thrust 

 premaxillary appears to have no analogue in other known odontocctes. 

 In some living genera {Pscudorca, Globiccphala) the lacrimal sends 

 up a thin, inconspicuous ridge-like process closely applied to the 

 curved anterior border of the supraorbital process and occasionally 

 extending over the rim of the process to the extreme anterior edge 

 of the dorsal surface. Such a structure might be interpreted as the 

 last trace of a backwardly spread portion of the lacrimal like that of 

 Xcnorophns which had been almost obliterated by a subsequent out- 

 ward-extending of the maxillary. In Stcnodclph'is the maxillary 

 shows a slight trace of terraced structure, but I do not find it present 

 in other living genera. The most natural explanation of this struc- 

 ture seems to be a depression of the rostrum subsequent to a backward 

 extension of the maxillary in a horizontal direction over the orbit. 



Another stage is found in all of the remaining known members 

 of the suborder, living and extinct, except Platanista, Physcter (with 

 its exrinct relatives), and Kogia; this statement naturally not apply- 

 ing to fossils so imperfectly preserved that the characteristic struc- 

 ures cannot be determined. The braincase has now enlarged and the 

 upper margin of the occipital has come into contact with the frontal ; 

 thus the postorbital constriction has been obliterated and the parietal 

 has been excluded from the vertex (pi. i, fig. la). The maxillary 

 extends backward over the frontal nearly or quite to the anterior 

 margin of the occipital. Laterally it may (pi. i, fig. la) or may not 

 spread out so as to cover practically the entire dorsal surface of the 



