452 PELMATOZOA. 



In one of these ancient types, from the Cambrian rocks of North 

 America, which Mr Walcott has doubtfully referred to the genus 

 Eocystites, there appears to be the exceptional character that the 

 pores are formed by the apposition of corresponding indentations in 

 the edges of contiguous plates (fig. 321, a). In the Ordovician 

 series, and particularly in strata of Llandeilo-Bala age, the remains 

 of Cystideans are sometimes abundant, and the group attains here 

 its maximum. In the Silurian period Cystideans are less abundant, 

 and in the Devonian formation only very few forms (Tiaracrinus, 

 Ateleocystites, Strobilocystites, &c.) are known to occur. Lastly, if 

 Codaster be removed to the Blastoids, the group of the Cystideans 

 is chiefly represented in Carboniferous deposits by the aberrant 

 genus Agelacrinus. 



As regards their classification, the Cystideans may be conveniently 

 divided into the three orders of the Aporitidce, Diploporitidce, and 

 Rhombiferi, according as the calycine plates are imperforate, are 

 pierced by yoked pairs of pores indiscriminately distributed, or 

 have their pores arranged in " pore-rhombs " ; and the characters 

 and principal genera of these three groups may be very briefly 

 glanced at here. 



Order i. Aporitid^e. — In this group are comprised all those 

 Cystideans in which the calyx-plates are destitute of pores. Of the 

 genera of this group Cryptocrinus (fig. 325, b) has a globular calyx, 

 composed of comparatively few plates arranged in a tolerably definite 

 manner, there being three basals, succeeded by two cycles of pen- 

 tagonal plates. The central mouth is, in perfect specimens, covered 

 by a vault of small calcareous plates, and the laterally placed anus 

 is provided with a "valvular pyramid." The stem and arms are 

 unknown, and the genus is confined to the Ordovician rocks. 

 Malocystites, from the Ordovician deposits of Canada, possesses an 

 indefinite number of plates in the calyx, and the mouth is excentric, 

 and is surrounded by radiating ambulacral furrows. In Anomalo- 

 cystites {= Ateleocystites, Billings) the calyx is also composed of an 

 indefinite number of plates, which are ornamented with transverse 

 striae. One side of the calyx is convex, while the other is flat or 

 concave, and the summit is provided with delicate armlets or pin- 

 nuke. The species of this genus range from the Ordovician to the 

 Devonian. The most remarkable type of the imperforate Cysti- 

 deans, however, is that presented by Agelacrinus and Edrioaster, 

 which have sometimes been regarded as constituting a special 

 family (Agelacrinidm). In the genus Agelacrinus (including Hemi- 

 cystites, Hall) the body (fig. 323, a) is in the form of a depressed 

 or convex disc, attached by the whole of its under surface to 

 some foreign body, and therefore devoid of a peduncle. The 

 upper or ventral surface of the disc is covered with numerous 



