FREE APPENDAGES TETRAPODA 275 



SiRENiA, in accordance with their aquatic Ufe, have a greatly reduced pelvis. 

 In the ancestral Eotherium all three bones are present, pubis and ischium bound- 

 ing a fenestra; and in Halitherium there is a rudimentary acetabulum and femur. 

 Halicore has but two bones — ilium and pubis — on either side, the former con- 

 nected with the sacrum by ligament. Manatus has merely a single triangular 

 bone on a side. 



Primates. — Most Lemurinte and Simias have a wide pelvis, in other lemurs 

 it is narrow. When wide, the pubis is broad and the symphysis is restricted to 

 this part. An acetabular bone is common in the young, fusing in various ways 

 in the different genera, but always excluding the pubis from the acetabulum. 

 The old-world apes often have ischial tuberosities ending in a broad flat surface 

 which is connected with the ischial callosities of the shin. 



Free Appendages 



The free appendages of Tetrapoda are very different from the fins 

 of fishes, but no doubt exists as to their general homology, though it is 

 difficult to trace details in the two groups, and what parts of the one 

 are what parts of the other is uncertain. The Tetrapod appendage 

 has to support the body weight, a necessity of the terrestrial life, and 

 this requires, not the general flexibihty of the fin, but a rigid support 

 and also joints permitting much motion, with muscles to hold the 

 parts firmly when occasion calls. 



As in fishes, the appendages arise as paired horizontal folds, extending over 

 several somites, and at first (especially in Amniotes) the early stage of ^n append- 

 age is a paddle with narrow base and a broader end evenly rounded at first, later 

 developing the (five) digits from the margin. Chondrificalion proceeds distally, 

 except for delay in the basipodal region. 



Pectoral and pelvic limbs are closely similar (homonomy), part 

 corresponding to part almost to details. Each has a proximal seg- 

 ment (upper arm or brachium, thigh or femur) called the stylopo- 

 diimi (fig. 242). Next is a zeugopodium (forearm or antebranchium, 

 cms or shank) and this is followed by the autopodium (hand or 

 manus, foot or pes). There is a single stylopodial bone-(humeius in 

 the arm, femur in the leg) and two zeugopodial bones, radius on the 

 preaxial side of the fore limb, ulna on the other, and tibia and fibula 

 in similar positions in the hind limb. 



Humerus and femur have a head, usually more or less hemispher- 

 ical, which fits in the socket (glenoid fossa, acetabulum) in the girdle. 

 Each usually has prominences near the head (tuberosities on the 



