Second species, H. hawesi is described from Morrison Formation of Colorado. This species is named after William Hawes who found the holotype.[4]
H. americana reached their maximum standard length (excluding fins) of 15.5 centimetres (6.1 in), while H. hawesi is much smaller, with standard length of 5.85 centimetres (2.30 in) (about 7.2 centimetres (2.8 in) total length).[2][4]
Formations that H. americana known are considered as marine environment,[3] while Morrison Formation where H. hawesi described shows freshwater environment.[4] Discovered specimens show evidence of predation upon the smaller species of fish Todiltia.[6]