82 



CETACEA 



ORDER IV 



constitutes the main part of the cranial roof. Through the shortening and 

 lateral compression of the parietals, the supraoccipital and interparietal of 

 the Odontocetes and some of the baleen whales meet the very short frontal, 

 which broadens laterally on both sides into large bony plates extending over 

 the orbits. The high occiput, sloping posteriorly, is formed entirely by the 

 supraoccipital. The deeply set squamosal (sq.) has a well-developed prominence, 

 looking forwardly and upwardly, which joins with the posterior wing of the 

 frontal and which serves as a support for the zygomatic arch, bounding the 

 elongated orbits, and which, in the case of the Odontocete whales, is thin and 

 rod-like (Ju.), and in the baleen whales is strongly developed. 



The very elongated, usually flattened and narrowed muzzle is composed 

 of the frequently asymmetrical premaxillaries, the maxillaries, vomer and 

 mesethmoid. The premaxillaries are either closely approximated in front 

 or else leave vacuities above, which broaden out posteriorly. At the hinder 

 end of these bones and adjacent to the frontals are the almost perpendicular, 

 or steeply inclined nostrils which function as blow-holes. The external nostrils 

 are overhung by short, well-developed nasals in the case of the baleen 

 whales ; but among Zeuglodonts the nasals are elongated ; Odontocete whales 

 have the nasals so reduced that frequently only a bony protuberance remains. 

 The embryos still retain the primitive structure. The backward displace- 

 ment of the nostrils has caused the extension of the maxillary and pre- 

 maxillary bones behind the orbits and the shortening of the frontals, 

 parietals and nasals, which are also crowded to the side of the skull. The 

 brain-case is bounded anteriorly by the almost perpendicular, broad meseth- 

 moid (7716.). All recent Cetacea have the maxillary enclosing the premaxillary 

 on the sides throughout its length except on the extreme tip of the snout ; the 

 fossil Zeuglodontidae and Squalodontidae have the premaxillary projecting 

 over the maxillary in front and set with teeth on its lower surface. Only a 

 few baleen whales and Physeteridae have a lachrymal bone. 



The inferior portion of the snout is formed principally by the horizontal 

 branches of the maxillary, though the vomer, a narrow bone lying in the 



median line between the two por- 

 tions of the maxillar}^, frequently 

 takes part in forming the roof of 

 the mouth. The short, strip-like 

 palatal bones bound the maxillary 

 bone behind and are in contact 

 posteriorly with the well -de- 

 veloped pterygoids, which meet 

 in the median line, and increase 

 the length of the bony palate 

 besides forming the walls of the 

 choanae. Of especial interest are 

 the auditory bones (periotic and 

 tympanic) (Fig. 110) placed between the squamosal and the exoccipital. They 

 are extraordinarily massive, are only loosely united with the other bones of 

 the skull, and therefore are easily lost in the fossil state. Isolated periotic 

 and tympanic bones (cetoliths) are found in almost all strata yielding Cetacean 

 remains, and ear-bones of whales have also been dredged from the floor of the 

 ocean. 



Fio. llu. 



Left periotic of Globicepluilns uncitUns from the Crag of 

 Orford, as seen from the side directed toward the tympanic. 

 a, Posterior, and h, anterior process ; c, hemispherical 

 portion. 



