CRINOIDS. 139 



Class I. — CEINOIDEA. 



The lowest division of the echinoderms and at the same time the one wliieh has 

 the fewest recent representatives is the Crinoidea. The fossil forms of this class are 

 very numerous iu the older rocks, and are commonly known as encrinites and stone- 

 lilies. The recent forms are so little known and so seldom seen by any except those 

 \\ho are familiar with the results of deep-sea explorations that they have received no 

 idHimon name. The greater portion of the species are attached to sub-marine objects 

 by a stem which, frequently, is very long, and made uj) of a scries of joints perforated 

 by a central canal. These joints are among the most numerous fossils in the older 

 rocks, and have received the name of St. Cuthbert's beads. This stem supports a 

 calyx (corresponding to the central disc of the star-fish) which has received its name 

 from its similarity to the calyx of a flower. From this calyx radiate the arms. Some 

 forms have the stalk persistent throughout life, while others possess it only in the 

 early stages, and in a few fossil forms it is said to be lacking in all stages. 



The class is usually divided into three orders, the Blastoidea, the Cysti<loa, and 

 the Brachiata, or true crinoids. The first of these is extinct, the second contains one 

 rt'cent form, and the tliird, until recently, was thought to be represented, with 

 one exception by free swimming forms. The recent deep-sea explorations have, 

 however, brought to light several sjiecies, some of considerable size and others much 

 smaller than some of the fossil forms. 



Order I. — BLASTOIDEA. 



The members of this extinct group of Crinoids were armless, were supported on a 

 short stalk, and had five double series of pinnules, one along each side of five radiating 

 grooves. The entire animal, in its fossil state, with the oral plates closed, looks like 

 a flower-bud. The most ancient form (Pentremites) is found iu rocks of upper Silu- 

 rian age, and the group is most abundant in the carboniferous. 



Pentremites has the ambulacral and ant-ambulacral regions nearly equal. The 

 calyx is composed of three basal plates, two of which are double. Above these lie five 

 plates deeply cleft above, and in the clefts lie the apices of the ambulacra, the oral 

 portions of which are included between the five interradials which surround the cen- 

 tral aperture. This is prolialily the mouth, and around it are four double pores, and 

 a fifth divided into three. Of these three the middle one is believed to be the anal, 

 while the other two, and the remaining jiairs are genital. Each ambulacrum consists 

 of two rows of small plates which are united in the middle line, and bear pinnules at 

 their outer ends. 



Order II. — CYSTIDEA. 



The Cystidea come near the crinoids, are usually furnished with arms, having 

 jointed pinnules, and have a short stalk. Caryocystites has no stalk and no arms, the 

 body being an angulo-spherical ball of solid plates. Several genera {Edrioaster, 

 At/elacrinites, Ilemiajstites), are also arndess and stalkless, but in form resemble such 

 a star-fish as Pteraster, except that they are more nearly circular. These forms have 

 five ambulacra, looking like the arms of an ophiurid placed in the midst of the disc, 

 and, like the more normal stalked cystid, they possess, in one of the interambulacral 



