"The holothurian genus Achistrum is subdivided, on the basis of the terminal loop, into the subgenus Achistrum sensu stricto and three new subgenera, Spinrum, Cancellrum, and Aduncrum. Their relationships are discussed. . . . Achistrum ranges from the Carboniferous to the middle Jurassic, and the possibility of its occurrence in the Cretaceous should not be disregarded, as no complete study concerning the holothuroids of this period has yet been made."