24 Mammalia. 



centrum, leaving a canal for the blood-vessels of the tail : 

 but towards the end of the tail they dwindle gradually in size, 

 lose their processes, cease to have an arch, and assume the 

 form of a double cone, resembling the phalanges of fingers. 

 Anchylosis is an exception (cf. xiii.) in this region. 



III. The Scapular Arch and Appendages. The anterior 

 pair of limbs is never absent in Mammalia. As is the case 

 with the vertebral bodies of Mammalia so is it with the limb 

 bones ; almost all of which have the articular surfaces, in the 

 growing state, supported on distinct plates, called epiphyses, 

 which usually coalesce with the rest of the bone at maturity. 

 The Scapula is generally present as an expanded plate of bone, 

 but is of varied width (cf iii. xi.), length (cf. iii. iv.), and 

 size (cf. X, 2.) : the pre-scapular ("supra- spinous") or anterior 

 fossa is almost invariably larger than the post-scapular (in- 

 fraspinous) fossa. The Coracoidy though developed from an in- 

 dependent osseous centre, generally coalesces with the scapula, 

 of which it appears as a small process (cf. xv.). The Clavicle 

 (= the OS furculare of Aves), when present varies much in 

 shape and size (cf. vii. ix. x. xi. xii.) ; it is generally united 

 to the acromion (cf. xiii.) by ligament, and is usually single 

 (cf. XV.) on either side. The whole of the scapular arch is 

 specially modified in the Monotremata (cf. xv.). The JTumerus 

 and bones of the fore-arm (antibrachial) and hand are specially 

 modified according as the animal lives in air or water, or bur- 

 rows beneath the surface of the earth. There is no power of 

 rotation of the fore limb in any hoofed quadruped. There 

 are usually two bones in the fore-arm, the radius and ulna, 

 which admit of various degrees of rotation. The Humerus is 

 generally straight (cf. vi. ) ; in swimming and fossorial ani- 

 mals it is very short (cf. iii. xi. xv.), otherwise it may be 

 described as characterised in the normal Mammalia by having 

 a head, tuberosities, deltoid crest, twisted shaft, epicondyloid 

 processes, and intermediate trochlear articular surface for 



