Diagnosis: Hippopotamyrus longilateralis can be distinguished by following characters: distance from dorsal fin origin to end of caudal peduncle 37.9-41.7% of standard length; predorsal length 61.2-64.9% of standard length; distance from tip of snout to anal fin origin 59.0-62.7% of standard length; caudal peduncle length 19.5-22.0% of standard length; head length 19.2-22.8% of standard length; snout length 45.7-54.8% of head length; body depth 20.3-24.5% of standard length; number of dorsal fin rays 18-21; number of anal fin rays 22-25; number of scales in a linear series along the lateral line row 74-80; number of scales around caudal peduncle 19-22; and bipolar, pentaphasic waveform of EOD pulse (Ref. 85059).

Description Head broadly rounded with a terminally positioned mouth, lower jaw protruding less forward than upper; body long and slender, laterally compressed (Ref. 85059). Dorsal fin origin set far back and behind that of anal fin, situated about two-thirds of standard length from snout, obliquely orientated with anterior portion of the fin higher than the posterior portion; number of dorsal fin rays 18-21; anal fin opposite dorsal fin but origin further anterior and also obliquely orientated, anteriorly lower and posteriorly higher, margin broadly rounded; number of anal fin rays 22-25 (Ref. 85059). Scales cycloid with reticulate striae, except in centre, extending anteriorly to operculum and pectoral fins; scales around caudal peduncle 19-22 (Ref. 85059). Caudal peduncle slender and subcylindrical over the entire length, usually slightly less than one-fifth in standard length; tail fin with broadly rounded lobes (Ref. 85059). EOD bipolar and pentaphasic, duration of the two main phases about 230 µs but whole pulse may last up to 5 milliseconds because of weak pre- and post-potentials (Ref. 85059).

Colouration: Dark brown with a vertical black band from dorsal origin that is only weak in most specimens (Ref. 85059). Colour in preservation dark to medium brown, often with milky-grey hue, especially where opaque 'mormyrid skin' is present (Ref. 85059).