II RIBS AND STERNUM ce! 
are persistently free for a very long period, and in some cases 
never become ankylosed with their vertebrae. But it should be 
noted that in this group there is no approximation to the state 
of affairs which exists in many lower Vertebrates, where there is 
a gradual transition between the ribs of the cervical and those of 
the dorsal region of the vertebral column ; for that of the seventh 
ribs in Monotremes is smaller than those which precede it. 
The Sternum.— All the Mammalia so far as is known possess 
a sternum. This is the bone, 
or series of bones (sternebrae), 
which les upon the ventral 
surface of the chest, and to 
which the ribs are attached 
below. .The development of Pat ee ii 
the sternum has been shown = i a 
to take place from the fusion of Oy 
the ribs below into two lateral 
bands, one on each side; the 
approximation of these bands 
forms the single and unpaired 
sternum of most mammals. 
Very considerable traces, how- 
ever, of the paired state of the 
sternal bones often exist; thus 
in the Sperm Whale the first 
piece of the sternum is divided 
into two by a longitudinal Fic. 18.—Sternum and sternal ribs of the 
aa! 7 i Common Mole (Talpa europaea), with 
division, and the second piece the clavicles (cl) and humeri (H); J, 
is longitudinally orooved. The Aamo sterni. Nat. size. (From 
2 s Flower’s Osteology.) 
development of the sternum 
out of the fused ends of ribs is shown in a more com- 
plete condition in some specis of Manis than in many other 
mammals. Thus in IL tricuspis the last ribs of those which are 
attached to the sternum are completely fused together into a 
single piece on each sidet As a general rule the last ribs 
which come into relation with the sternum do so only in an 
imperfect Way, being simply firmly attached at their sides to, 
but not fused with, the last ribs which are definitely articulated 
with the sternum. Contrary to what is found in lower Verte- 
1 Ehler’s Zool. Miscellen. i. 1894. 
