XIII 



PHYLUM CHORDATA 



379 



ret 



(f.trs.), is left between the three bones of the shoulder-girdle. The 

 furcula is an investing bone and represents fused clavicles and 

 interclavicle. 



Equally characteristic is the skeleton of the fore-limb. The 

 Immerus (Fig. 1045, 7m.) is a large, strong bone, with a greatly 

 expanded head and a prominent ridge for the insertion of the 

 pectoral muscle. In it, as in 

 all the other long bones, the 

 extremities as well as the shaft 

 are formed of true bone. The 

 radius (ra.) is slender and 

 nearly straight, the ulna stouter 

 and gently curved. There are 

 two large free carpals, a radiale 

 (ra.'} and an ulnare (ul/), and 

 articulating with these is a 

 bone called the carpometacarpus 

 (cp.mtcp.) consisting of two 

 rods, that on the pre-axial side 

 strong and nearly straight, that 

 on the post-axial side slender 

 and curved, fused with one 

 another at both their proxi- 

 mal and distal ends ; the 

 proximal end is produced, pre- 

 axially, into an outstanding 

 step-like process. The study 

 of development shows that 

 this bone is formed by the 

 union of the distal carpals 

 with three metacarpals (Fig. 

 1046), the second and third of 

 which are the two rod-like 

 portions of the bone, the first 

 the step-like projection. Ar- 

 ticulating with the first meta- 

 carpal is a single pointed 

 phalanx (Fig. 1045, ph. 1) ; the 

 second metacarpal bears two 

 phalanges, the proximal one 

 (ph.2 r ) produced post-axially into a flange, the distal one (ph.2") 

 pointed; the third metacarpal bears a single pointed phalanx (ph.3). 



The pelvic girdle (Fig. 1038) resembles that of no other Vertebrate 

 with the exception of some Dinosaurs. The ilium (il.) is an immense 

 bone attached by fibrous union to the whole of the syn-sacmm and 

 becoming ankylosed with it in the adult. It is divisible into 

 pre-acetabular and post-acetabular portions of approximately equal 



ra' 



FIG. 1045. Columba livia. Skeleton of the 

 left wing, cp.mtcp. carpo-metacarpus ; hu. 

 humerus ; ph. 1, phalanx of first digit ; ph. 2', 

 ph. 2", phalanges of second digit ; ph. 3, 

 phalanx of third digit ; pn. for. pneumatic 

 foramen, ra. radius ; ra'. radiale ; ul. ulna ; 

 ul'. ulnare. 



