CYSTOIDEA. 



455 



related to Echinosfihcerites, but the calyx is more of an ovoidal form, 

 and the plates are ridged or striated. Allied genera are Palaocystites 

 and Comarocystites, both from the Ordovician rocks of North 

 America, and the latter remarkable in having a small number of 

 pinnate arms. 



In a second group of the Rhombiferi, the pore-rhombs are con- 

 fined to the sides of the calyx, and the upper surface is formed of 

 imperforate plates, while the arrangement of the calycine plates 



Fig. 325. — A, Echinosphcerites atiranthnti ; b, Cryfitocrinus lavis ; c, Echinoencrinns Senken- 

 bergi; d, Echinoencrinus (?) annatus ; e, One of the " pectinated rhombs " of the last, enlarged; 

 f, Holocystites cylbtdricus. v, Valvular anal pyramid. All the specimens are viewed from one 

 side, (a, b, and c are after von Buch ; d is after Edward Forbes ; and f is after Hall.) 



generally is to a greater or less extent radial. A good example of 

 this group is afforded by the genus Caryocrinus (fig. 326, a), which 

 is widely distributed in the Silurian rocks of North America. In 

 this genus there is a long and cylindrical column, which carries an 

 ovoid calyx, the upper surface of which exhibits the articular facets 

 to which free-jointed armlets or pinnulae were attached. The lateral 

 calyx-plates show externally lines of pores radiating from their centres, 

 and an internal examination shows that these pores are connected 

 by canals into pairs, so as to form a series of "rhombs." Hemicos- 

 mites (fig. 318) is an allied genus, which is found in the Ordovician 

 and Silurian deposits of Europe. 



Finally, in a third group of the Rhombiferi are included forms in 

 which the calyx-plates are generally more or less definite in number, 

 and more or less clearly arranged in a quinary manner, while the 

 " pore-rhombs " are limited in number, and are often so disposed that 



