t)6 ]V<ilh!r Sf<i/>lf'i/ : 



toed sloth, the .inimal presents a long II. with the middle of the back 

 at the lowest part, and the head in a position that is more or less 

 erect. The two-toed sloth has a body and limbs that are lnn<jf and 

 lithe enou«rh to gather its food without the neck elongation that has 

 (iccurred in the three-toed sloth, and in its peculiar position, together 

 with the fact that it has abandoned quadrupedal gait, it would not 

 be surprising if dissection should reveal that its lung has risen into 

 the neck and cervical ribs have formed. 



The three-toed sloth is said by Wiedersheim to have cervical ribs 

 upon the seventh neck bone. A skeleton of this animal in the Mel- 

 bourne Museum does not show them, and Owen, who went very care- 

 fully into the question, only depicts rib-stumps on the eighth and 

 ninth bones. But it is not surprising that Wiedersheim has found 

 cervical rib structure in the three-toed sloth. In the normal mam- 

 mals, costal processes are absent from the seventh bone, and this 

 permits flexion of the neck on the chest, an extensive movement where 

 neck curvature is great. As the Brndypus, by its peculiar gait, 

 abolished the curvature of its neck, it may have become necessary to 

 stiffen up the seventh bone to harmonise it with the sixth above and 

 the eighth below, so that the neck may be of proper sti'ength in its 

 various parts. 



Clavicles and Flight. — The Pterojms or flying-fox, or fox-bat, is a 

 mammal that has acquired the power to fly, yet its neck type is true 

 to mammalian characteristics. This stability of type appears to be 

 due to several causes : this animal walks on the ground in quadru- 

 pedal fashion, and not after the manner of birds, on the pelvic limbs : 

 and in developing its air-planes it has had to do without the great 

 help that birds derive from the coracoid bones, consequently those 

 changes that occur in birds through the presence of the coracoid 

 bones, are not to be seen in the flying-fox. Although the clavicles of 

 the flying-fox are greatly developed, they can do no more than partly 

 compensate for the absence of the coracoids, because the position of 

 the clavicle is anterior and external to the central and interior posi- 

 tion which the coracoids, if present, would occupy. Tlic clavicles play 

 no part in the formation of cenncal ribs, but by holding the scapula 

 off the thorax, they afford the arm the means of executing a wider 

 range of rotary movement. 



If vestigial structures possess the ability to come liack to func- 

 tional activity, the corat'oid pi'ocesscs of this flying manunal should 

 re-establish the coracoid bones ; instead of that, the elavicle, by over- 

 development, throws the pectoral limbs from the ventral to the 

 lateral thoracic jiosition. Tlie coracoid process remains tmaffo-cted 

 in spite (if the demands that tiiglit makes upon coraeoid bones. If 



