THE SKELETON 43 
and 23 in Perissodactyla. The highest and quite exceptional 
numbers are in the Two-toed Sloth (Cholepus) 27, and the Hyrax 
30. The prevailing number of rib-bearing vertebre is 12 or 13, 
any variation being generally in excess of these numbers. 
Sacral Vertebre.—The sacral region offers more difficulties 0. 
definition. Taking the human “os sacrum” as a guide for 
comparison, it is generally defined as consisting of those vertebra 
between the lumbar and caudal regions which are ankylosed 
together to form a single bone. It happens, however, that the 
number of such vertebrae varies in different individuals of the 
same or nearly allied species, especially as age advances, when a 
certain number of the tail vertebre generally become incorporated 
with the true sacrum. Other suggested tests—as those vertebrae 
which have a distinct additional (pleurapophysial) centre of ossifica- 
tion between the body and the ilium, those to which the ilium is 
directly articulated, or those in front of the insertion of the ischio- 
sacral ligaments—being equally unsatisfactory or unpractical, the 
old one of ankylosis, as it is found to prevail in the average 
condition of adults in each species, is used in the enumeration of 
the vertebre in the following pages. The Cetacea, having no iliac 
bones, have no part of the vertebral column modified into a 
sacrum. 
Caudal Vertebre.—The caudal vertebre are those placed behind 
the sacrum, and terminating the vertebral column. They vary 
in number greatly—being reduced to 5, 4, or even 3, in a most 
rudimentary condition, in Man 
and in some Apes and Bats, and 
being numerous and powerfully 
developed, with strong and com- 
plex processes, in many mammals, 
especially among the Edentata, 
Cetacea, and Marsupialia. The 
highest known number, 46, is 
possessed by the African Long- 
tailed Pangolin. Connected with 
the under surface of the caudal 
vertebree of many mammals which 
have the tail well developed are 
certain bones formed more or less 
like an inverted arch, called chev- 
ron bones, or by the French os en — Fra. 12.— Anterior surface of fourth 
a These are always situated caudal vertebra of Porpeise (Phoceenc. cate: 
. munis). 8s, Spinous process ; m, metapophy- 
nearly opposite to an interverte- sis; t, transverse process ; h, chevron bone. 
bral space, and are generally artic- 
ulated both to the vertebra in front and the vertebra behind, but 
sometimes chiefly or entirely either to one or the other. 
