obturator foramen, that is between the pubis and ischium with which the real obturator or pubic foramen is merged, occurs in the Theriodontia (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 between the two bones, the pelvis still retaining its primitive plate-like character with only a small median pubo-ischiatic vacuity. But this will not explain the thyroid vacuity in Pteranodon and Nyctosaurus of the Pterosauria (Fig. 118), since it is inconceivable that these reptiles had an unbroken descent from forms without a median vacuity.
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.
In no reptiles is the pelvis more aberrant than in the Crocodilia (Fig. 121). So characteristic is its structure that it at once distin-
