. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. Pareiasaurian Studies.—Part VI. 299 they are, in addition, very massive, whereas in others they must, in comparison, be called " ; These three bones—the ilium, ischium, and pubis—are very firmly united to each other in ankylosis to form an os innominatum. The two innominate bones are hardly less firmly united to each other than are their constituent elements. The sym- physis, which is thickened in all the forms and ventrally forms a strong carina, is very markedly thickened in the for
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. Pareiasaurian Studies.—Part VI. 299 they are, in addition, very massive, whereas in others they must, in comparison, be called " ; These three bones—the ilium, ischium, and pubis—are very firmly united to each other in ankylosis to form an os innominatum. The two innominate bones are hardly less firmly united to each other than are their constituent elements. The sym- physis, which is thickened in all the forms and ventrally forms a strong carina, is very markedly thickened in the forms with narrow pelvis (text-fig. 1). Dorsally the girdle is completed by a number of sacral ribs, which attach the girdle to the vertebral column. To withstand the strain of this attach- ment a closer union between the vertebrae con- cerned is brought about, and it is found that in some forms two, and in others three or four, vertebrae are ankylosed to form a sacrum. The sacral ribs are not all equally developed. In some species the strongest rib is the first, whilst in others it is the second. This would indicate Text-fig. l.—Longitudi- ,-,.„. , t nal sections through the that dirierent vertebrae are concerned, as it symphysis, x ^. must be taken that the strongest rib is attached (^Bradysaurus baini to the primitive single sacral vertebra. In those (3533)- (b) ET^^' x ° . saurus schwarzi (8034). forms where the first sacral rib is the strongest, its vertebra is taken to represent the first sacral; where the second is the strongest, the first rib belongs to a lumbo-sacral vertebra ; where the fourth sacral rib is attached to a vertebra not ankylosed to its pre- decessor, this is taken to represent a caudo-sacral vertebra. The third rib is always attached to a vertebra ankylosed to its predecessor, so that it is taken to be a true sacral vertebra which has gone through a caudo-sacral stage. In all the Pareiasaurs the main sacral attach- ment of the ilium lies, as in
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