THE PECTORAL AND PELVIC GIRDLES 



147 



obturator foramen, that is between the pubis and ischium with which 

 the real obturator or pubic foramen is merged, occurs in the Therio- 

 dontia (Fig. 119), Anomodontia, and later pterodactyls (Fig. 118 d). 

 The formation of a thyroid vacuity in the theriodonts may be due to 

 the gradual increase in size of the pubic or true obturator foramen 

 and its recession backward, as in the Dromasauria, till it finally lies 



Fig. 121. Pelvic girdle and sternum: Alligator (Crocodilia). A, pelvic girdle, from the 

 right; B, the same, from above, showing sacrum; C, the same, from below, with 

 parasternals; D, sternum and interclavicle. One half natural size. 



between the two bones, the pelvis still retaining its primitive plate- 

 hke character with only a small median pubo-ischiatic vacuity. But 

 this will not explain the thyroid vacuity in Pteranodon and Nycto- 

 saurus of the Pterosauria (Fig. 118), since it is inconceivable that 

 these reptiles had an unbroken descent from forms without a 

 median vacuity. 



In no reptiles is the pelvis more aberrant than in the Crocodilia 

 (Fig. 121). So characteristic is its structure that it at once distin- 



