Description of a new species of Phenacostethus (Atheriniformes: Phallostethidae) endemic to Kalimantan Selatan, Indonesian Borneo, reveals deep mtCOI divergence among miniature species Author Parenti, Lynne R. Author Lumbantobing, Daniel N. Author Haryono text Raffles Bulletin of Zoology 2023 2023-09-29 71 553 571 journal article 10.26107/RBZ-2023-0042 2345-7600 C4DA8175-9DFF-42A0-A0ED-1E574B8138E7 Genus Phenacostethus Myers, 1928 Type species. Phenacostethus smithi Myers, 1928 , by original designation and monotypy. Differential Diagnosis. Phenacostethus differs from the genus Phallostethus in having a lower oral jaw that protrudes beyond the upper jaw and a reduced second ctenactinium (priapial bone) which is prominent and serrated in Phallostethus . Phenacostethus has fewer anal-fin rays (14–15 as opposed to 26–28), second dorsal-fin rays (5–7 as opposed to 8–10) and vertebrae (33–35 as opposed to 40) than in Phallostethus (Parenti, 1986: table 1). Phenacostethus has a first dorsal fin with one unsegmented ray versus first dorsal fin absent in Phallostethus . Phenacostethus and Phallostethus differ from all other phallostethines ( Neostethus and Gulaphallus ) in having a priapium with a prominent, hooked toxactinium and a shield-like pulvinular pad, structures present in other priapiumfishes but not as well-developed. Composition and distribution. Four species, including the new species described herein ( Table 1 ): Phenacostethus smithi Myers, 1928 , fresh and brackish waters, Thailand , Cambodia , Sumatra, Peninsular Malaysia , Sarawak , and Brunei ; Phe. posthon Roberts, 1971a , fresh and brackish waters, Thailand and Peninsular Malaysia , and Sumatra; Phe. trewavasae Parenti, 1986b , freshwater, Sarawak , Malaysian Borneo, and Brunei ; Phe. sikat , new species , freshwater, Kalimantan Selatan, Indonesian Borneo.