24. SACRUM AND CAUDAL SERIES CHAP. 
number of vertebrae fused together and supported by the pelvic 
girdle is characteristic of the mammals, but is not found in the 
Cetacea and the Sirenia, where functional hind-limbs are wanting. 
‘Strictly speaking, the sacrum is limited to the two or three 
vertebrae whose expanded transverse processes meet the ilia. 
But to these are or may be added a variable number of vertebrae 
withdrawn from both the lumbar and the caudal series, which 
Fie. 12.—Lepus cuniculus. Tnnomin- Fic. 13.—-Anterior surface of fourth 
ate bones and sacrum, ventral aspect. caudal vertebra of Porpoise (Pho- 
acet, Acetabulum ; 7, ilinm ; isch, coena communis). x4. h, Chevron 
ischium ; obf, obturator foramen ; bone ; m, metapophysis; s, spin- 
pub, pubis ; sacr, sacrum 3 sy, sym- ous process ; ¢, transverse process. 
physis. (From Parker and Haswell’s (From Flower’s Osteology.) 
Zoology.) 
unite with each other to form the massive piece of bone which 
constitutes the sacrum of the adult. 
The caudal vertebrae complete the series. They begin in as 
fully developed a condition as the lumbars, with well-marked 
transverse processes, etc.; but they end as no more than 
centra, from which sometimes tiny outgrowths represent in 
a rudimentary way the neural arches, ete. Very often the 
caudal vertebrae are furnished with ventral, generally V-shaped, 
appendages, the chevron bones or intercentra.’ These are 
' Intercentra are but rarely met with anterior to the caudal series. Mr. Parsons 
has, however, recorded their occurrence in the lumbar vertebrae of Atherura. 
