272 



VERTEBRATE SKELETON 



anatomy), the medial boundary of the fenestra being osseous in the 

 adult, a condition paralleled in recent reptiles only in Chelonia. 

 Three (or four) centres of ossification occur in the cartilage, and in 

 many species all three bones enter the acetabulum, which is imper- 

 forate, except in Monotremes, where the acetabular cup has a complete 

 boundary wall. In other groups the wall has a gap between ischiadic 

 and pubic parts for the round hgament (1. teres) connecting the 

 head of the femur with the bottom of the cup. 



The adult pelvis differs from that of most reptiles and resembles 

 that of Therapsida and Amphibia in the ventral and posterior direc- 

 tion of the ilium from its connexion with the 

 sacrum, and this general direction is repeated in 

 ischium and pubis, the latter the least incUned of 

 the three, while the ischium often appears as a 

 direct continuation of the ilium (fig. 300). In 

 the more primitive orders (Marsupials, Ungu- 

 lates, most Rodents) both pubis and ischia are 

 Fig. 297. — Early connected in the middle Hne by symphysis, an 



stages of pelvis of sheep . . 



(Mehnert). ii, ilium; intcrpubic Cartilage intervemng m some Insec- 

 is, ischium; p, pubis. ^-^^^^gg^ Edentates and bats, recalling the epi- 



pubis of non-mammals. More commonly there is no ischiadic 

 symphysis, and in many bats, Soricidae and Talpidae (fig. 299) even the 

 pubes are connected only by Hgament, lost in the prepared skeleton. 



Primitively the ihum is long and slender and not infrequently 

 is prolonged in front of the sacral articulation — the union of the 

 ihum with one or two sacral ribs of the true sacrals, and by cartilage 

 or ankylosis (some bats and Edentates), with the synsacrals. The 

 chief iliac modifications are the development of a crest on the dorsal 

 margin (large in Edentates) and sometimes a lateral crest parallel 

 to the axis, which in primitive forms ends in a posterior ventral spine. 



The ischium is usually larger than the pubis with which it is 

 connected 'on either side of the fenestra. The ischia are rarely 

 (some Edentates and bats) connected with the vertebral column, 

 their upper margins then uniting with several postsacral vertebrae. 

 Occasionally a process Hke a postischium is present, and there may 

 be strong ischiadic tuberosities on the posterior border of the bone. 

 The pubes are directed downwards and more or less backwards. 

 They are parallel or may diverge behind in bats and some Insectivores, 

 and some genera of several orders (Marsupials, Rodents, etc.) have a 



