viii.] CETACE A. EDENTATA. 113 



In the aberrant Physeteridce^ including the Sperm Whale, 

 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. 59. 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} u, 

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

 A'o^ia, 14. 



Among the EDENTATA the Sloths have very numerous 

 ribs (from 15 to 24 pairs, the latter number in Cholcepus 

 didactylus, of which 12 are true ribs). In the anterior part of 

 the thorax the sternal ribs are firmly ossified, and indistin- 

 guishable 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 (Fig. 45) 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 

 vertebral 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 (Myrmecophaga) has been already 

 described (see p. 102). The ribs of the small climbing Two- 

 toed Anteater (Cydoturus didactylus] are remarkable for a 

 thin lamelliform expansion of their hinder border, over- 

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



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