Two new nematode species of the genus Cosmocephalus Molin, 1858 (Spirurida: Acuariidae), with an amended generic diagnosis and an identification key to Cosmocephalus spp.

Cosmocephalus podicipis n. sp. is described from Podiceps cristatus from the vicinity of Ramsar, Iran (type host and type locality) and from Podiceps nigricollis from the Bulgarian Black Sea coast. Cosmocephalus pelecani n. sp. is described from Pelecanus conspicillatus from Mansfield, Victoria, Australia. Comparative materials of three other species of Cosmocephalus were examined and described. These are C. obvelatus (based on specimens from Larus argentatus from Bulgaria), C. jaenschi (based on a paratype from Phalacrocorax carbo from Australia) and C. capellae (based on a male nematode from Tringa totanus from Bulgaria, a new geographical record). Cosmocephalus faridi from Pelecanus onocrotalus from Egypt, which has been considered by previous authors as a synonym of C. obvelatus, is validated. Results of SEM observations are presented for C. podicipis and C. obvelatus. In all the species examined, the presence of a single ventral ridge was recorded in males anterior to the caudal bursa. The generic diagnosis is amended to include more comprehensive information on the morphology of the studied species. Currently, the genus Cosmocephalus includes 7 species. Cosmocephalus imperialis is recognised as a species inquirenda. Cosmocephalus argentinensis is proposed as a junior synonym of C. obvelatus. An identification key to the species of Cosmocephalus is presented.