There exist fundamental differences between Crossocheilus oblongus and Epalzeorhynchos siamensis.
A certain ichthyologist is of different opinion.
I wonder why.

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Crossocheilus oblongus Kuhl & Van Hasselt
for comparison





Images by Ronald de Ruiter

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Crossocheilus siamensis  (Smith, 1931)
is a name by Banarescu for
Epalzeorhynchos siamensis Smith, 1931

Its holotype is
USNM 090302 Epalzeorhynchus siamensis.





Images of the holotype by Sandra Raredon
Copyright Division of Fishes, Department of Vertebrate Zoology, Museum of Natural History, Smithsonian Institution,
Washington, D.C.

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Crossocheilus oblongus  Kuhl & van Hasselt in van Hasselt 1823

Its alleged holotype or syntype according to Roberts is
RMNH 2640 Crossocheilus oblongus.





Images of the holotype or syntype by Ronald de Ruiter









Images of the holotype or syntype by Koos van Egmond
Copyright Department of Ichthyology, National Museum of Natural History,
Leiden

Its holotype designated by Alfred according to Tan & Kottelat is
RMNH D1755 Crossocheilus oblongus.





Images of the holotype by Ronald de Ruiter
Copyright Department of Ichthyology, National Museum of Natural History,
Leiden

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Crossocheilus langei  Bleeker 1860

Its holotype is
BMNH 1866.5.2.106 Crossocheilus langei.








Images of the holotype by James Maclaine
Copyright Department of Zoology, British Museum of Natural History,
London

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Crossocheilus cobitis (Bleeker, 1853)
is a name by Bleeker, 1860 for
Lobocheilos cobitis Bleeker, 1853


Its syntype is
BMNH 1866.5.2.54. Crossocheilus cobitis.







Images of the syntype by James Maclaine
Copyright Department of Zoology, British Museum of Natural History,
London

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Crossocheilus pseudobagroides Duncker, 1904


Its holotype is
ZMH 373 Crossocheilus pseudobagroides.





Image of the type by Dr. Ulrike Strecker
Copyright Dr. Ulrike Strecker, Zoological Museum of the University of Hamburg and GBIF - Global Biodiversity Information Facility