PHYLUM CHORDATA 



397 



synovial joints. Ossified uncinates are nearly always present, and 

 usually become ankylosed to the vertebral ribs. 



What may be considered as the normal type of sternum is a 

 broad plate, concave dorsally from side to side, and produced 

 ventrally into an antero-posterior keel which is ossified from a 

 distinct centre (Fig. 1002, A, os. 1}. The posterior edge of the bone 

 is either entire (D) or presents, on each side of the keel, one or two 

 more or less deep notches (A, B) or foramina (C). In the Ratita? 



A . 



FIG. 1002. Sterna of various Birds. A, Callus (common Fowl, young) ; B, Turdus (Thrush) ; 

 C, Vultur (Vulture) ; D, Procellaria (Petrel) ; E Casuarius (Cassowary), ant.lat.pr. 

 anterior lateral process ; car. carina ; d. clavicle ; cor. coracoid ; fan. fontanelle ; fur. furcula ; 

 obi. lat.pr. oblique lateral process ; os. paired ossification of sternum in E ; os. 1, carinal ossifi- 

 cation in A ; os . 2, os. 3. lateral ossifications ; post. med. pr. posterior median process ; 

 post. lat. pr. posterior lateral process ; pr. cor. pro-coracoid ; scp. scapula ; sp. spina sterni. 

 (A and E after W. K. Parker ; B, C, and D, from Bronn's Thierreich.) 



(E) the keel is either absent or reduced to the merest vestige, and 

 there is no trace of the carinal ossification in the young. External 

 to the coracoid grooves the anterior edge of the sternum is pro- 

 duced into larger or smaller antero-lateral processes (ant. lat.pr.) : 

 in the Emu these are of great size and are closely applied to the 

 pericardium. 



It was upon the characters of the raft-like sternum that the 

 group Ratitse was founded, but the difference between them and 

 the CarinataB in this respect is not absolute, the ratite condition 



