490 



MAMMALIA. 



is there a distinct interclavicle (episternum). While the number 

 of cervical vertebrae is almost constantly seven, that of the 

 dorsal vertebrae is subject to a greater variation. As a rule 

 there are thirteen, sometimes twelve dorsal vertebrae ; but there 

 is a less number in some bats and armadillos, while there are 

 fifteen or more in some animals. The horse has eighteen, the 

 rhinoceros and elephant nineteen to twenty, and the three-toed 

 sloths have twenty-three to twenty-four. The lumbar verte- 

 brae, which have long lateral processes in place of ribs, are 

 usually seven in number. The number rarely sinks to two as 

 in Ornithorhynch'us and the two -toed sloths, and still more 

 rarely, rises to eight or nine {Stenops). The sacral vertebrae, 



"^t 



Fig. 261. — Skeleton of the I. ion (after Giebel). C calcaneum ; Cp carpus ; F fibula ; Fe femur ; 

 H humerus ; Jl ilium ; Js ischium ; Mc metacarpus ; Mt metatarsus ; P patella ; P pubis 

 R radius ; Sc scapula ; St sternum ; T tibia • Ts tarsus ; U ulna. 



which vary in number from two (marsupials) to four, more rarely 

 nine (Armadillo), are firmly united with one another, and by 

 their transverse processes (with the rudiments of the ribs) with 

 the iliac bones. In whales and Sirenia there is no union of ver- 

 tebrae to form a sacrum. The caudal vertebrae, which vary 

 considerably in number and mobility, become narrower towards 

 the end of the axis of the body, and often (kangaroo and ant- 

 eaters) possess inferior spinous processes (chevron bones or 



