PENTREMITES, pen'tre-mPtez (Neo-Lat.
nom. pl., from Gk. irlyrc petite, five Lat. remus, oar). A genus of fossil echinoderms typical of the class Blaatoidea. The blast...ids were crinoid-like animals that lived in abun dance in the sea of late Paleozoic time. They resembled the crinoids in respect of the gen eral plan of their organization, but they dif fered in that the calyx has only 13 principal plates arranged in very regular order, general ly forming a bud-shaped head (whence the name), and in the absence of well-developed arms. In place of the arms there were very efficient pinnules that screened the food from the water. The calyx is usually small, pear shaped, ovate, or globose in form and more or less pentangular in section. The stem is seldom found attached to the calyx. About 20 genera and 150 species of blastoids are known, by far the larger proportion of which are American only.
The blastoids appear to have been derived from some of the more regular cystideaus of Ordovi cian time. The earliest member of the group is
Troostocrinus, an elongate slender species from the Niagara. group of Tennessee. Elwecrinus (Nueleoerinus) verneuili is a well-known index fossil of the Lower Devonian limestones of tho Central States. The blastoids attained great prominence in the Subcarboniferous of North America, especially in the Saint Louis and Kas kaskia limestones of the Mississippi Valley. Here the most prominent genus is Pentremitcs, which seems to be unknown in Europe, where it is represented in the Devonian rocks by an ances tral form Pentremitidea. See Von Zittel and Eastman, Textbook of Paleontology, vol. i. (New York and London, 1900) ; and Etheridge and Carpenter, Catalogue of the Blastoidcu in the Geological Department of the British Museum.. The latter work contains a full bibliography to the literature of the group. See CRINOIDEA ; ECHINODERAIATA.