440 



PELMAT0Z0A. 



jenus Hyocrinus, 



which is in some respects related to the Palseo- 

 crinoids. In this genus the calyx-plates are thin, 

 and the cup consists of both basals and radials, 

 the former being only three in number. The 

 radials are five in number, and each carries a 

 single undivided arm. The arm-joints are united 

 by "syzygy" into groups of two or three, the 

 terminal joint of each group bearing a long pin- 

 nule. There are five large "oral" plates round 

 the mouth. The stem consists of short cylin- 

 drical joints, the articular surfaces of which are 

 simple or slightly striated. 



Family 5. Plicatocrinidce. — This family in- 

 cludes only the Jurassic genus Plicatocrinus, 

 which is considered by Zittel as nearly allied to, 

 or identical with, Hyocrinus. Basal plates are, 

 however, said to be absent ; while the arms are 

 short, and the joints are not united by " syzygy," 

 and are provided with short pinnules. 



Family 6. Apiocrinidce. — In this family the 

 calyx is regular, and the upper stem-joints are, 

 typically, wider than those below, the upper 

 part of the column thus passing gradually into 

 the base of the cup. The basals are five in 

 number, and are surmounted by three cycles of 

 radials, each cycle being normally pentamerous. 

 The stem is expanded at its base for attachment 

 to foreign bodies, or is furnished with branching 

 roots or cirri (fig. 314), and the apposed faces 

 of its component joints are marked with radial 

 striae. The earliest types of this family (Afiio- 

 crinus and Millericrinus) appear in the Jurassic 

 rocks, and became extinct in the Lower Creta- 

 ceous period. 



In the genus Apiocrinus, comprising the so-called 

 " Pear-encrinites," there is a long cylindrical stem, 

 rooted to foreign bodies by an expanded base (fig. 

 314). The upper joints of the stem are narrower, as 

 regards vertical height, than the lower ones, while 

 they are widened out laterally, so as to pass gradually 

 into the massive pyriform cup. The uppermost joint 

 of the stem is much enlarged, and carries the five 

 basals, which are in turn succeeded by three rows of 

 radials. The species of Apiocrinus are found in 

 the Jurassic and Lower Cretaceous rocks. In the genus Millericrinus, 

 with a very similar geological range, the upper stem-joints are only 



Fig. 314. — Apiocrinus 

 Roissyanus. Middle Oo- 

 lite (Jurassic). 



