ORDER CETACEA. 



1301 



or double aperture usually near the vertex of the skull. The bones 

 are usually of a spongy nature, and contain a large amount of oil. 

 The cervical region of the vertebral column is always very short ; 

 and the seven component elements may be partially or completely 

 fused together, while the odontoid process of the axis when present 

 is short and blunt, and may be entirely want- 

 ing. None of the vertebrae unite to form a 

 sacrum. The lumbar and caudal vertebrae 

 are large and numerous, and from the ab- 

 sence of zygapophysial articulations allow of 

 a large amount of motion in the hinder part 

 of the body ; the presence of chevron-bones 

 distinguishes the caudals from the lumbars. m - 

 The terminal epiphyses of the vertebrae do 

 not unite with the centra till the animal is 

 fully adult. The cranium presents peculiar 

 features which it will not be necessary to 

 describe here • although it may be mentioned 

 that the usually small nasals are generally 

 brought up near to the vertex, and that there 

 is a more or less elongated rostrum in ad- 

 vance of the external nares, formed by the 

 premaxilla, maxilla, mesethmoid, and vomer. 

 Clavicles are absent ; the scapula and hu- 

 merus are well developed and freely mov- 

 able upon one another, but the anterior 

 bones of the limb admit of only a very 

 slight amount of movement. There are 

 usually five digits in the manus (fig. 1171), 

 but these may be reduced to four ; the 

 phalangeals are unique among Mammals in 

 exceeding the number of three to a single 

 digit, and also in being furnished with epi- 

 physes. The pelvis is represented by a pair 



of styliform bones, which are regarded as Sf)? k "SedS zV ^Radi^ : 

 the ischia ; and there are occasionally small «. Ulna ; c, Carpus (the letter 



.~ . ., . , placed on the scaphoid or 



OSSlficatlOnS Or Cartilages representing the radiale); mi-m 5 , Metacarpus; 



aborted bones of the proximal part of the mJ^ eT J halangeals - (After 

 hind limb. Teeth are usually present, but 



are very variable in number and size. The dentition in existing 

 forms is homceodont and monophyodont, but it was heterodont 

 in the extinct Zeuglodontida of the Eocene. 



The Cetacea are not known with certainty before the Eocene, 

 and are most abundant in the later Tertiary periods. The denti- 

 tion of the Eocene Zeuglodontidce indicates that the order has been 



Fig. 1 171.— Dorsal aspect of 

 the left pectoral limb of the 



