VERTEBRATA. 419 



The girdles in a general way usually present three parts, 

 cartilaginous in lower types, bony in higher. These are the 

 following : — 



Although these two girdles can be thus directly com- 

 pared, they become very dissimilar in higher types. The 

 pectoral girdle has a membrane-bone, the clavicle^ which 

 replaces the precoracoid. This joins the sternum, and 

 hence the pectoral girdle becomes connected ventrally with 

 the axial skeleton; but in the pelvic girdle the dorsal 

 element or ilium becomes attached to the vertebral column, 

 forming a sacrum, and the pubes and ischia tend to fuse 

 ventrally. 



The limbs are in most Pisces formed on the type called 

 ichthyopterygium, consisting of one or more basal pieces 

 {cf. Skate) bearing a row of distal elements ox fin-rays. (See 

 Fisces.) In the other Vertebrata the limbs are of the type 

 called a cheiropterygiuni or pentadactyle limb. In describ- 

 ing this type we may first explain the following terms : — 

 Both limbs in their supposed primitive position hang 

 down on either side of the body, and if we draw an 

 imaginary axis down the centre of the limb, certain parts 

 of the limb are nearer the head, these being termed pre- 

 axial, whereas those nearest to the hind-end of the animal 

 are called postaxial. Again, the part of the limb which is 

 closest to the body is termed the proximal end and the 

 part furthest away the distal end, and generally any point 

 described as distal lies further out than one called proximal. 

 The typical cheiropterygium has a single proximal limb- 

 bone, called in the fore-limb the humerus, in the hind-limb 

 the femur. Then follow two distal limb-bones in each case; 

 the preaxial is the radius in the fore-limb, the tibia in the 

 hind-limb ; whilst the postaxial are respectively the ulna 

 and the fibula. The small bones which follow are the 

 carp alia or wrist-bones and the tarsalia or ankle-bones. 



