442 



PELMATOZOA. 



arranged in a floriform manner, each petaloid sector having from five 

 to eight large teeth at the sides. There are only three radials, and the 

 arms rarely divide more than thrice. The genus Metacrinus, only known 

 by living forms, differs from Pentacrinus chiefly in the possession of 

 from four to six radials, the characters of the joint-faces being the same 

 in both genera. Lastly, the Mesozoic genus Extracrinus (fig. 290) is 

 separated from Pentacrinus by the possession of a ring of underbasals, 

 and by the much more frequent subdivision of the arms. The faces of 

 the stem-joints in this genus are also different, the petaloid sectors being 

 linear, and having delicately crenulated edges. In all three genera the 

 visceral mass is surrounded by a plated perisome, which unites the lower 

 part of the rays and arms. 



Family 9. Marsupitidce. — This family comprises free Crinoids, 

 in which the larval peduncle is lost in the adult. A " centro- 

 dorsal " plate is present, but this does not 

 bear cirri. The calyx may be monocyclic 

 or dicyclic, and there are no definite in- 

 terradial plates. The only genera in- 

 cluded in this family are Marsupites and 

 Uintacrinus, both of which are confined 

 to the Cretaceous period. 



The genus Marsupites comprises the so- 

 called " Tortoise - encrinites " of the Chalk, 

 and is characterised by the possession of a 

 globular calyx, composed of large thin plates, 

 carrying bifurcated uniserial arms. The calyx 

 is dicyclic, the underbasals, five in number, 

 surrounding a central pentagonal plate, which 

 has usually been regarded as an enlarged top 

 stem-joint (" centrodorsal "). There is, how- 



Fig. 315. — Side-view of the 

 calyx and bases of the arms of 

 Marsupites ornatus, from the 

 Chalk. 



ogous with the suranal plate of Salenia, and 

 therefore a " dorsocentral " plate. There are 

 five basals and three rows of radials, but regular interradials are not 

 developed. The ventral surface of the calyx is vaulted, and covered 

 with small calcareous plates ; and the anus is sub-central. The genus 

 Uintacrinus resembles Marsupites in its general form, and in the pos- 

 session of a pentagonal "centrodorsal" (?) which does not carry cirri. 

 On the other hand, the globular calyx is monocyclic, and there are ten 

 arms, the bases of which are united laterally by numerous perisomic 

 plates, as in the Pentacrinida. 



Family 10. Comatulidce. — This family of Crinoids comprises the 

 " Feather-stars," all of which (except Thiolliericrmus) lose the larval 

 peduncle, and are therefore free in the adult condition (fig. 284). 

 The base of the calyx is closed by a " centrodorsal " plate (fig. 

 285, cd), which represents the enlarged top stem-joint, and upon 

 which jointed cirri are developed. The calyx is apparently " mono- 

 cyclic," but underbasals have recently been discovered in the larva 



