212 



VERTEBRATE SKELETON 



reduced nasals are upright on the hinder side of the opening. The frontals 

 are greatly shortened and extend far laterally, forming a supraorbital plate to 

 which the maxilla contributes in Odontocetes. Since frontals and supraoccipital 

 meet, and with the slight use of the jaws in chewing, there is no sagittal crest in 

 living species, but Zeuglodon, with more specialized teeth, has a crest. 



In some whales the exoccipitals exclude the basioccipital from the foramen 

 magnum. The maxilla in Zeuglodon (fig. 226) is normal, extending back only 

 to the frontal; in Odontocetes it is widened behind, covering much of the supra- 

 orbital plate of the frontal, but in 

 Mystacocetes there is no such over- 

 lapping. The premaxillas, while 

 long, form only a small part of 

 the border of the mouth in exist- 

 ing whales, much more in Zeug- 

 lodon. The zygomatic arch is 

 slender, the zygomatic bone lying 

 ventral to the orbit and meeting 

 the imperforate lacrimal; it may 

 fuse with other bones. Occasion- 

 ally the squamosal meets the 

 postorbital process of the frontal, 

 separating orbit and temporal 

 fossa externally; the orbit being 

 low on the side of the head. The 

 relations of nerves to orbito- and 

 alisphenoid are primitive, the 

 latter being imperforate, and fre- 

 quently the optic nerve passes 

 through the orbital fissure. 



The ethmoid is without olfac- 

 tory foramina, the nerve being 

 degenerate; the nasal conchae 

 persist in Mystacocetes, but are 

 degenerate in Odontocetes. 

 Many Mystacocetes have the pal- 

 atal processes of the pterygoids 

 meeting behind the palatines, car- 

 rying the choanae farther back 

 than in most mammals. The 

 petrosal is connected by ligaments to the adjacent bones; it does not project into 

 the cranial cavity, and together with the tympanic with which it is fused, easily 

 separates from the rest of the skull, forming the 'cetoliths' found in deep-sea 

 dredging. The tympanic part forms a very thick bulla. The halves of the 

 lower jaw are cylindrical in Mystacocetes, compressed from side to side in 

 Odontocetes. They are united in front by articulation or ankylosis in Odonto- 

 cetes, merely by ligament in Mystacocetes. In correlation with the slight 



Fig. 225. — Cranium of Legenorhynchus. e, 

 ethmoid; /, facial; m, maxilla; 71, nasal; o, 

 occipital; p. parietal; pm, premaxilla; s, squa- 

 mosal; s, zygomatic. 



