94 THE RIBS. [chap. 



Hyperoodon and various forms of Ziphioids, the ribs are 

 connected to the vertebrae throughout the greater part of the 

 series by both head and tubercle ; but a few of the most 

 posterior have only a single point of attachment in con- 

 sequence of the changes which take place in the condition 

 of the transverse processes of the vertebrae, described at 

 p. 53. In this family the sternal ribs are either permanently 

 cartilaginous, or very imperfectly ossified. The Hyperoodon 

 has but 9 pairs of vertebral ribs, the smallest number 

 known in any Mammal, the Sperm Whale (Physeter) 11, 

 of which the last is quite rudimentary; Ziphius 10, and 

 Kogia 14. 



Among the Edentata, the Sloths have very numerous 

 ribs (from 15 to 24 pairs). In the anterior part of the thorax 

 the sternal ribs are firmly ossified, and indistinguishable 

 from the vertebral ribs (at least in adult age), but posteriorly 

 they are separated from the latter by a less perfectly ossified 

 intermediate rib. 



In the Armadillos the ribs are comparatively few (10 to 

 12 pairs), and are broad and flat, the first extremely so. The 

 first sternal rib is very short and incorporated with the ver- 

 tebral rib, but the others are very strongly ossified, and 

 articulated by synovial joints with the sternum, with each 

 other, and with the vertebral ribs. 



The peculiar double articulation of the sternal ribs with 

 the sternum in the Anteater {Myrmecophagd) has been already 

 described (see p. 83). The ribs of the small climbing Two- 

 toed Anteater {Cydothurus didactylus) are remarkable for a 

 thin lamelliform expansion of their hinder border, over- 

 lapping the succeeding rib. This animal has 15 pairs, the 

 great Anteater 16, the Cape Anteater (Orycteropus) 13. 



The Marsupialia have nearly always 13 pairs of ribs; 

 the Koala (Phascolarctos) with but 11, and the common 



