http://www.nhm.ac.uk/hosted-sites/iczn/code/index.jsp wrote:11.9. Species-group names.
11.9.1. A species-group name must be a word of two or more letters, or a compound word (see Article 11.9.5), and, if a Latin or latinized word must be, or be treated as,
188.8.131.52. an adjective or participle in the nominative singular (as in Echinus esculentus, Felis marmorata, Seioptera vibrans), or
184.108.40.206. a noun in the nominative singular standing in apposition to the generic name (as in Struthio camelus, Cercopithecus diana), or
220.127.116.11. a noun in the genitive case (e.g. rosae, sturionis, thermopylarum, galliae, sanctipauli, sanctaehelenae, cuvieri, merianae, smithorum), or
18.104.22.168. an adjective used as a substantive in the genitive case and derived from the specific name of an organism with which the animal in question is associated (as in Lernaeocera lusci, a copepod parasitic on Trisopterus luscus).
11.9.2. An adjectival species-group name proposed in Latin text but written otherwise than in the nominative singular because of the requirements of Latin grammar is available provided that it meets the other requirements of availability, but it is to be corrected to the nominative singular if necessary.
adrianus wrote:"Asteroidea" is really a feminine singular adjective ("asteroideus -a -um") referring to the species or class (the words for species/class are feminine in Latin—"asteroidea species/classis"). In English the adjective is used collectively, but not in Latin.
adrianus wrote:Even in modern naming species is singular: // Secundum systemma nomenclaturae modernum, singulis numeri est nomen speciei