VERTEBRATA. 267 



with a ventral piece, the coracoid; the latter articu- 

 lates ventrally with the median breast-bone or ster- 

 num, and is overlaid by a membrane-bone called the 

 clavicle or collar-bone. Between the clavicle lies an 

 additional median bone, the interclavicle ; this is es- 

 pecially characteristic of reptiles, and is not often 

 seen elsewhere. Other subordinate elements are 

 sometimes distinguished. The pelvic girdle consists 

 correspondingly of three pieces on each side, dorsally 

 the ilium, which is connected with the spine, and ven- 

 trally the ischiuui and pubis. The fore-limbs are 

 respectively attached at the joint between the scapula 

 and the coracoid, and the joint between the ilium and 

 ischium. While the higher reptiles have usually five 

 digits, it may be noticed that in some extinct reptiles 

 a more fin-like type of limb was present. 



The Eeptiles, although they resemble the lower 

 groups of the vertebrate series in being cold-blooded, 

 differ from them in possessing a structure called the 

 amnion, which enwraps the embryo in the egg. They 

 share this character in common with Birds and 

 Mammals, and the three classes are therefore some- 

 times grouped under the name of Amuiota, while 

 the Pishes and Amphibia (Ichthyopsida) are referred 

 to under the name of Anamniota. The same is true 

 of another embryonic structure called the Allantois, 

 and the group names Allautoidea and Anallau- 

 toidea are therefore similarly employed. 



The distinguishing external feature of Reptiles is 

 the great development of the armature of the skin, 

 consisting of scales or of bony dermal plates. The 



