Question: Melicytus and Astrophytum, both are cacti?

Context articles: Astrophytum asterias
Cactus
Melicytus novae-zelandiae subsp. centurionis
Astrophytum
Astrophytum coahuilense
Astrophytum caput-medusae
Astrophytum capricorne
Astrophytum ornatum
Melicytus
Infraspecific name

Astrophytum asterias is a species of cactus in the genus "Astrophytum", and is native to small parts of Texas in the United States and Mexico.
 Common names include sand dollar cactus, sea urchin cactus, star cactus and star peyote.
A cactus (plural: "cacti", "cactuses", or "cactus") is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales.
 The word "cactus" derives, through Latin, from the Ancient Greek κάκτος , "kaktos", a name originally used by Theophrastus for a spiny plant whose identity is not certain.
 Cacti occur in a wide range of shapes and sizes.
 Most cacti live in habitats subject to at least some drought.
 Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on earth.
 Cacti show many adaptations to conserve water.
 Almost all cacti are succulents, meaning they have thickened, fleshy parts adapted to store water.
 Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place.
 Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves.
 As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade.
 In the absence of leaves, enlarged stems carry out photosynthesis.
 Cacti are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north—except for "Rhipsalis baccifera", which also grows in Africa and Sri Lanka.
Melicytus novae-zelandiae " subsp. "
 centurionis is a flowering plant in the violet family.
 It is a subspecies of "Melicytus novae-zelandiae", known in New Zealand as Coastal Mahoe.
 The subspecific epithet honours the military Captain James Doran McComish (1881–1948), who made several visits in the 1930s to collect plants on Lord Howe Island.
Astrophytum is a genus of six species of cacti, native to North America.
Astrophytum coahuilense is a cactus species belonging to the "Astrophytum" genus.
 "A. coahuilense" are native to Mexico, specifically the south-western regions of Coahuila and Durante.
 Although superficially similar to "A. myriostigma", the two species bear different flowers, fruits and seeds.
 "Astrophytum coahuilense" can be grown from seeds and, though they tend to be slow growing, are amenable to botanical cultivation.
Astrophytum caput-medusae (synonym "Digitostigma caput-medusae") is a species of cactus native to Mexico, specifically the state of Nuevo León; the plan is reported to grow wild only at a single location.
 This species differs from the conventional star-shaped phenotype associated with other Astrophytum members.
 The plant is characterized by a cylindrical, reduced stem with triangular or cylindrical tubercles producing yellow flowers with orange perianth sections.
 Propagation by seed, tissue culture or via grafting have all been reported.
 The species is considered critically endangered by the IUCN due to the plant's limited growth range, destruction by livestock and over collection of wild specimen by plant collectors
Astrophytum capricorne, also known as the Goat's horn cactus, is native to the Coahuila regions of Northern Mexico.
 The common name of Goat's horn cactus corresponds to the species identifier "capricorne" ("capri" meaning goat and "corne" meaning horn) referring to the curved spines that are said to resemble a goat's horns.
 This species is designated as of "least concern" by the IUCN as the species is widely distributed in the Chihuahuan desert.
 This species is grown, typically from seed, as an ornamental plant as it produces large, attractive yellow flowers with red central regions.
 Some cultivars bear white flecking on the plant body while others lack this feature.
 Prominent cultivars of this species include: Astrophytum capricorne “crassispinoides”, Astrophytum capricorne v. crassispinum, Astrophytum capricorne v. minus, Astrophytum capricorne v. niveum, Astrophytum capricorne v. senile, Astrophytum capricorne v. aureum.
Astrophytum ornatum, also called bishop's cap, is endemic to the Central Plateau of Mexico.
 It is the largest and tallest species of "Astrophytum".
Melicytus is a genus of flowering plants in the family Violaceae.
 "Hymenanthera" is a synonym.
In botany, an infraspecific name is the scientific name for any taxon below the rank of species, i.e. an infraspecific taxon.
 (A "taxon", plural "taxa", is a group of organisms to be given a particular name.)
 The scientific names of botanical taxa are regulated by the "International Code of Nomenclature for algae, fungi, and plants" (ICN).
 This specifies a 'three part name' for infraspecific taxa, plus a 'connecting term' to indicate the rank of the name.
 An example of such a name is "Astrophytum myriostigma" subvar.
 "glabrum", the name of a subvariety of the species "Astrophytum myriostigma" (bishop's hat cactus).