In frog, the vertebra with an anterior convex surface (i.e., double convexties) is
atlas
urostyle
8th vertebra
9th vertebra
D.
9th vertebra
The 9th or sacral vertebra is acoelous and highly specialised the anterior face is convex for this give greater strength them, it would be given if its anterior end are hollowed out the transverse process are stout and downwardly directed. The ileum of the pelvic girdle articulates with the transverse process of 9th vertibrae.
The first phylogenetic system of plant classification was proposed by
Engler
Engler and Prantl
Eichler
Wetlstein