Flower shape in adeniums is a continuum from very narrow-petaled (extreme star shape) to nearly perfect circles. The shape is complicated by other characteristics such as quilled (rolled) or ruffled petals. Shapes on a single plant can also vary with weather and cultural conditions. I like the definitions of the four shapes that I made up below, but it is often difficult to classify many flowers on the border between two types.
Star-shaped flowers have such narrow petals that the free limbs (the part beyond the fused floral tube) barely if at all overlap with each other. They usually also have pointed petal tips.
Nearly all flowers of Adenium multiflorum are star-shaped.
Adenium crispum 'Famous Ancestor' flowers are typical for the species: star-shaped and quilled, with nectar guides far onto the petals.
Adenium "obesum" 'Grumbley White' flowers are very star-shaped. It's from a wild collection. More modern whites have round to circular flowers.
'Colorful and Wonderful', a hybrid of "obesum" and crispum.
Flower of an Adenium somalense growing in Bangkok. The petal tips are fairly rounded; this flower could be called a semi-star.
An old A. crispum hybrid with very narrow petals.
'Black Raspberry', an "obesum"-crispum hybrid
'Crimson Star' is approaching a semi-star because of the wider petals.
'Success' is also nearly a semi-star. This plant has such weak stems that they hang vertically.
Semi-star shaped flowers usually have pointed petals, but the petals are wide enough to overlap for up to half of their free lengths.
Adenium "obesum" 'Black Ruby'
Adenium "obesum" 'Daeng Siam'
The flowers of the "obesum"-crispum hybrid 'Classical' are a bit pointy. The petal overlap is rather good, but the flower outline is not round.
Most Adenium socotranum have semi-star shaped flowers. There is little petal overlap, but the tips are round.
'Rainbow' is an "obesum"-swazicum hybrid. The wide petals overlap well, but the pointed petals give it a semi-star shape.
Round flowers have considerable petal overlap, and usually rounded petal tips.
Adenium "arabicum" 'Hansoti Dwarf'
Adenium "obesum" 'Amiability'
'Pink Elegance', an "obesum"-swazicum hybrid, backcrossed to "obesum".
It's more difficult to describe the shape of
Adenium "obesum" 'White Wave' because of the ruffled petals. It might be circular if the flower were flat.
'White Jade Peony', an "obesum"-swazicum hybrid with very flat petals. The tips are somewhat pointy, but the flower does not look semi-star shaped.
Circular flowers have rounded petal tips and very wide petals that overlap for well over half their lengths and nearly fill the circle they circumscribe. Perhaps there are not yet any truly circular adenium flowers, but the following cultivars at least come close.
Adenium "arabicum" 'Yemeni Giant' has nearly circular flowers. This is excellent for a first-generation selection from a wild plant.
Adenium boehmianum tends to have circular flowers.
'Evelyn Marie', a complex "obesum"-swazicum hybrid.
Adenium "obesum" 'Home Run'
'Taiwan Beauty' (aka 'Beauty of Taiwan') is a hybrid of "obesum" with either swazicum or boehmianum.
The ruffled petals of this Adenium "obesum" 'MAD 335' create an almost circular outline. Unfortunately, this blond color is unstable. A few of the normally purplish flowers bleach to this shade when they're a week old.