Type species of the genus: Bathycrinus gracilis Thomson, 1872.
Synonymy: Bathycrinus Thomson, 1872: 772; Ilycrinus Danielssen & Koren, 1877: 45; A.H. Clark, 1908 a: 236; 1915: 153–154; 1917: 389–390; Bathycrinus Gislén, 1938: 14 –15; Roux, 1977: 27–28; Rasmussen, 1978: T 844; Roux et al., 2002: 815, 818, 822; Mironov, 2008: 135–136; Hess, 2011 a: T 153.
Remarks. It is difficult to distinguish Atlantic species using only external morphological characters as proposed by A.H. Clark (1908 a) in his key to species in the genus Bathycrinus. B. aldrichianus Thomson, 1876 and B. gracilis Thomson, 1872 have similarly very serrated crowns profiles, and B. gracilis displays sharper median crests on the division series. However, A.M. Clark (1977: 160) suggested that “possibly further material may indicate that only one species can be recognized”. Both B. carpenteri Danielssen & Koren, 1877 from the Norwegian Sea and the Arctic Ocean and B. australis A.H. Clark, 1907 b from the South Atlantic and Southern Ocean differ in having smooth rounded brachials. Characters other than external morphology are required to clearly distinguish among these different Bathycrinus species. As pointed out by Roux (1977), highly derived characters present in articulations of xenomorphic stalks allow a better distinction between taxa and can be used for taxonomic attribution of isolated stalks. The Bathycrinus stalk exhibits two divergent ontogenetic profiles of columnals: the first one develops rigidity in the mesistele, the second provides flexibility in the dististele (Duco & Roux 1981, Fig. 2). Below, we provide additional information on stalk synarthries and their ontogeny which can be used as discriminating characters in taxonomy.
For details on characters of external morphology used to distinguish B. aldrichianus and B. australis, see Gislén (1956) and A.M. Clark (1977). Moreover, new distinctive characters in bathycrinids such as pinnule architecture have been proposed by Mironov (2000, 2008).