Bathyembryx
Bathyembryx istiophasma, the pallid sailfin, is hypothetical species of fish observed by William Beebe on 11 August 1934. He describing seeing the species twice during the same dive at depths of  and  near the coast of Bermuda. Background

The “bathysphere,” as termed by Beebe was a new yet primitive invention. It was a rounded steel enclosure with space adequate for two people, its external layer being . On the side, there was a single window made of fused quartz,  across and  thick. It was fitted with a heavy steel door that had to be bolted on. With no manoeuvrability, the navigation of this steel submersible was solely dependent on the ship it had been attached to. Beebe had no camera brought with him to these great depth, and instead described the species in detail to Else Bostelmann, an artist who proceeded to illustrate his findings. From 1930 to 1934, this submersible was used by Beebe in his deep-sea expedition. The encounter

Beebe had begun to fathom what he had seen. Within moments, he had prepared a description for the new creature. He had stated it was large for a deep-sea fish, reaching a length of “two feet” at the very least. The fish was completely non-luminous, and had a small eye as well as a decently-sized mouth. The pectoral fins were long and wide but filamentous. Two features on the sailfin stood out, according to Beebe. He described the colour as sickly, pale and olive-coloured. In his own words, “the hue of water-soaked flesh”. The caudal fin was reduced, much like a “button”. The “vertical” fins which are assumed to be the anal and dorsal fin, extended greatly beyond its body. He admitted that he did not see the pelvic fins, and thus the illustration subsequently produced lacked them." Beebe called the fish Pallid Sailfin and assigned the scientific name Bathyembryx istiophasma, whose etymology he explained as: “…a Grecian way of saying it comes from the deep abyss and swims with ghostly sails.” He placed it in the family Cetomimidae. Status of existence

Of the five new fish described by Beebe during the Bathysphere dives, none of them were confirmed to exist. The existence of all fishes were supported by his colleague Otis Barton, who descended with him in the submersible. See also 

 Bathysidus
 Bathysphaera

References

Controversial taxa
Fish described in 1934