Variations and Abnormalities

It is quite usual to find fusions of various kinds between successive vertebrae, the most usual places being between the fifth and sixth and between the atlas and the vertebra next behind it. One interesting abnormality that does occasionally occur is the presence of the sacral diapophyses on the ninth vertebra, leaving a postsacral vertebra quite free from the urostyle (Plate 131, fig. 6). The ninth vertebra in these unusual cases is found to be formed of two halves joined by cartilage dorsally and ventrally, but to have both pre- and postzygapophyses. The postsacral vertebra is fairly small and has backwardly directed transverse processes, but otherwise is quite normal. The vertebral column in front of the ninth vertebra is clearly normal. This forward displacement of the sacrum is by no means uncommon among Amphibia. Zaharesco (1935) has made statistical analyses of this condition (and also that of spinal nerve variation) in Rana and concludes that even in that genus one still finds an evolutionary trend towards the shortening of the presacral region of the vertebral column.