The Mouth lies centrally on or below the tegmen, and grooves lead to it from the bases of the arms. The Anus is typically excentric, and is interradial in position (see below); that is, it occurs between two of the arms. In some extinct genera, the dome bears one aperture, that of the anus, which then occurs almost centrally, and at times on the end of a tube or "proboscis." In these cases the mouth is concealed beneath the dome, and the brachial grooves run through the wall, and are prolonged as little canals towards the centre, where they reach the mouth. The plates composing the calyx are grouped in several series from the base upwards to the region of the mouth. The lowest plates, meeting in the centre of the base, are 2 to 5 in number (commonly 5), and are termed Basals. They are often hidden in fossil specimens by adhesion to the upper stem-joints. Sometimes, however, the base is formed of two cycles of plates, an upper one, the true Basals, in this case sometimes styled Parabasals; and a lower cycle, alternating with the upper, and termed Infrabasals (see fig. 122). Next above the basals or the parabasals, and in either case alternating with them when the base has petagonal symmetry, is the cycle of the Radials, commonly 5 in number; vertically above these the arms of the crinoid rise. On each plate of this primary radial series one or more similar plates may stand (fig. 121), so that each arm may be supported on a vertical row of several ossicles, which are commonly entitled first, second, third, &c., radials (see "Arms" below). In several important genera these radial series are in contact laterally; but in other types there are plates or groups of plates intercalated between them, such plates being styled Interradials. In relation to the calyx as a whole, however, the basals or the parabasals are also "interradial" in position; the infrabasals, when present, are "radial" (see fig. 122). Anal Interradial Group.-This group commonly contains more plates than the others, and, on its continuation over the oral surface of the calyx, bears the anus. Frequently, interradials are found in no other portion of the calyx. * Arms. The ossicles composing these are all styled Brachials. Dr. P. H. Carpenter regards the members of the radial series * "Anatomical Nomenclature of Echinoderms," Ann. and Mag. of Nat. Hist., 6th ser., vol. vi. (1890), p. 15; F. A. Bather, "British Fossil Crinoids," ibid., vol. v., p. 313; and "Suggested Terms in Crinoid Morphology," ibid., vol. ix., p. 51. See also the terminology in Wachsmuth and Springer, "Revision of the Palæocrinoidea," Proc. Acad. Nat. Sci. Philadelphia, 1879 (pub. 1880), p. 249. Also Bather, Geol. Mag., 1898, p. 318. The lower above the first radial as all belonging to the arms. cycles of his brachials thus correspond to the old second and higher cycles of radials; these Dr. Carpenter styles Costals (fig. 121). Any interradials between these thus become styled Interbrachials. Above the costals the arm often bifurcates, further dichotomous division taking place in many genera. The free stems and branches of the arms are sometimes formed of one vertical row of ossicles ("uniserial;" fig. 122), sometimes of two in contact, the ossicles alternating in the two rows ("biserial;" fig. 121). On the inner surface of the arms a groove leads down from their tips to the upper part of the calyx. Pinnules.-Small arm-like processes, also formed of calcareous ossicles, set in many genera on both sides of the grooves that run down the arms. In living crinoids these bear the reproductive elements. The Stem is composed of a row of ossicles, placed vertically on one another, their articulating surfaces being variously ribbed and grooved. A central canal runs down through them all. The Crinoidea form a considerable portion of some limestones, the scattered ossicles of their stems, with their circular crosssections and often radial markings, having given rise to the name "entrochal marble." The abundance of stems at some horizons, apart from crowns, has often been remarked on, and it has been suggested that certain genera (as Actinocrinus) possessed the power of casting off their stems at particular stages of their growth. On the other hand, it has been pointed out that calyxes, unless at once filled with mud on the death of the animal, run much greater risk of destruction than the more solid stems, the ossicles of the calyx being scattered too widely for easy recognition. While, on the whole, the more modern types of crinoids are marked out by the smallness of the calyx in proportion to the arms, by a general absence of interradials, and also of a prominent ventral sac or dome, yet the division of the group into "Neocrinoidea" and "Palæocrinoidea" can be no longer maintained. At the present time the classifications of specialists in this refined branch of palæontology cannot be regarded as having reached even a resting stage; consequently the genera here selected, while showing an interesting range of structure, are not placed under any system of subdivisions. Encrinus (fig. 121).-Calyx rather shallow; 5 small infrabasals and 5 large parabasals present, the former generally hidden by traces of the stem; 5 radial series composed of three cycles, the ossicles of the two upper cycles being now regarded by most authors as belonging to the arms (first order of brachials, styled costals). From these arise simply bifurcating arms, which have pinnales, and which are commonly formed of two rows of ossicles (an exceptional feature in a Neozoic form). Upper surface of the calyx solidly roofed over. Stem long; its ossicles are radially grooved on their articulating surfaces. Trias. Pentacrinus.-Calyx very small in proportion to the arms; 5 basals (sometimes 5 infrabasals and 5 parabasals); 5 radials, above each of which lie 2 ossicles (radials or costals). The arms are formed of one row of ossicles, and bifurcate again and again, with long and abundant pinnules. Stem long, with numerous little jointed lateral processes; in cross-section it is sometimes rounded, but commonly appears like a five-rayed star, the indentations between the rays being deep or shallow. The articulating surfaces of the ossicles of the stem always bear a pattern of five oval markings, which radiate symmetrically from the central canal. These stem-ossicles form very familiar fossils. Trias to Recent. Common in the Lias. Fig. 121.-Encrinus liliiformis (Muschelkalk). The infrabasals and parabasals lie almost horizontally, and are invisible. Above these are seen the radials, each supporting two costals. The arms bifurcate, and ultimately become biserial. Apiocrinus.-Calyx narrowing slightly above; its plates, including the interradials, are fitted into one another to form a solid wall. 5 basals, alternating with which are the 5 radials, each bearing two large costals (2nd and 3rd radials); the basals rest on a circular plate, perhaps formed by the union of five infrabasals. Below this plate the stem commences, at first equal in diameter to the calyx, then contracting, and then becoming very gradually wider towards its rooted base. Hence above the narrowest part of the stem rises an egg-shaped or pear-shaped body, the upper half of which is the true calyx, the lower half being formed by the highest ossicles of the stem. Stem circular in cross-section. The arms are formed of a single row of ossicles, and bear pinnules. They bifurcate only once or twice. Ventral sac known. Lias to Lower Cretaceous. Actinocrinus.-Calyx, including its dome, rather ovoid; 3 basals, 5 radials, with 2 costals (2nd and 3rd radials) above each; interradials (interbrachials) present, with more numerous plates in the anal group. The upper surface of the calyx is formed by a fairly high convex dome of plates, sometimes bearing near the apex an anal tube. Arms repeatedly divided; composed of two rows of ossicles, and arising from five protuberances at the base of the dome, so as to appear to emerge about midway between the apex and base of the ovoid "head." Stem round, the central canal appearing five-rayed in cross-section. Gotlandian to Carboniferous; especially the latter. Platycrinus.-Calyx formed of 3 basals, 5 fairly tall and vertical radials, 5 much smaller costals, and one interradial (or, rather, interbrachial) in each interspace between the arms; there may be, however, 3 plates in the anal series. Calyx roofed over as in Actinocrinus, with or without anal tube. Arms repeatedly bifurcating, composed at first of one row of ossicles, and later of two. Stem-ossicles often elliptical in cross-section. Gotlandian to Carboniferous; especially the latter. Ichthyocrinus.-Calyx formed of 3 small infrabasals (sometimes not visible on exterior), 5 small parabasals, and 5 radials. 2 to 3 cycles of costals. Usually no interradials. Arms numerous, uniserial, and forming at their base a seemingly solid structure with the calyx. No pinnules known. Gotlandian to Carboniferous. Fig. 122-Cyathocrinus (Gotlandian). Showing infrabasals, parabasals, and radials. Uniserial arms. Cyathocrinus (fig. 122).-Calyx cup-like; 5 infrabasals; 5 large parabasals, forming part of the side-wall of the calyx; 5 radials; interradial plates occur only in the anal area. Upper surface with a greatly elongated and probably flexible dome. Arms long and repeatedly bifurcated, composed of one row of ossicles; no pinnules. Stem round. Gotlandian to Permian; especially the former. Heterocrinus.-Calyx small, somewhat cylindrical. 5 minute infrabasals, at times seemingly absent; 5 parabasals; 5 radials, with 2 costals above each. One or more of the radials is formed of two plates, united by a horizontal suture. Interradials in the anal group, and prolonged upwards as a ridge. Arms long, uniserial, with strong pinnules. Ordovician. Ventral sac known. B. BLASTOIDEA. In these forms, entirely Palæozoic and extinct, the calyx frequently resembles a closed flower-bud, having no free arms, an ovoid contour, and only a short stem. Running from the summit of the calyx down its sides are five elongated areas, the ambulacral (or pseudo-ambulacral) areas, which remind the observer of the well-known ambulacral areas on an echinoid. These are commonly seen as depressions, leaf-like in shape, or at times straight-sided. Specimens have been found in which the areas still bear pinnules, thus resembling crinoid-arms turned back and down over the calyx. A row of pores occurs down each side of the ambulacral areas, these openings being in reality interspaces between little lateral plates. The pores communicate with delicate canals, a bundle of which, called a hydro-spire, runs up internally on either side of the median line of the area, and opens at the summit of the calyx. Here there are usually five openings (spiracles), each representing two series of canals from adjacent ambulacral areas. The mouth is central, and the anus lies between the two posterior spiracles. The calyx is mainly composed of five plates (arising from a basal cycle), each of which is deeply notched above to receive the downward turned apex of an ambulacral area. These plates may therefore be regarded as radials. Pentremites.-Calyx bud-shaped, narrower above; radials large. Ambulacral areas rarely reaching to the base. Short round stem. Gotlandian to Carboniferous; especially the latter. Granatocrinus.-Calyx resembling Pentremites; but the ambulacral areas extend down to the base, and the radials are small, the five upper plates (interradials) between them being large. Carboniferous. The common "Pentremites" ellipticus thus becomes referred to Granatocrinus. C. CYSTIDEA. In this group, the Cystideans, there is a spherical or ovoid calyx, sometimes with pinnulated lateral ambulacral grooves |