Eclimothuridee, a family of Sea-urchins of which the type- genus is the chalk fossil Echinothuria. The best-known genera, however, are the living Asthenosoma and Phormosoma, which are mostly deep-sea dwellers. The most striking character of the family is that the tests are flexible and the plates imbricate; the spines are small and covered in membrane, and the apical system (q.v.) is rudimentary. These are all primitive characters, but the family is in all probability degenerate rather than ancestral. An "echinothuroid" form is probably attained by representatives of the different orders. The genus Pelanechinus from the Corallian rocks is usually referred to this family.