CRIXOIDEA. 283 



formed by crenated ridges arranged in a flower-like manner. 

 The principal genus is Pentacrinus (including Extracritms), 

 which made its first appearance in the Trias, is abundantly 

 represented in the Jurassic, is found in less numbers in the 

 Cretaceous and Tertiary, and still survives at the present 

 day. 



In the Apiocrinidce, or " Pear-Encrinites," the column is 

 long and rounded, and is rooted inferiorly to the sea-bottom. 

 The upper column-joints gradually increase in diameter till 

 they attain the width of the calyx ; or, if this does not occur, 

 the basal plates of the calyx are fused, so as to appear like 

 the highest joint of the stem. In the typical members of 

 the family the calyx consists of five basals and three cycles 

 of radials, but there are no inter-radials or parabasals. The 

 type genus of the family is the Jurassic Apiocrinus (fig. 

 169); Guettardicrinus, and Millericrinus are also Jurassic; 

 the curious genus Bourgueticrinus is Cretaceous and Tertiary ; 

 and Rhizocrinus (fig. 150) is found living at the present day. 



Intermediate between the ordinary stalked Crinoids and 

 the typical free sessile forms (such as Comatula) is the family 

 of the Holopidce, in which the animal is permanently fixed, 

 though there is no stem. In the living Holopus, the type of 

 the group, the basals, with the first, and probably also the 

 second, radials, are fused together, " forming the wall of a 

 tube-like body-chamber, which is cemented beneath to the 

 foreign body to which the Crinoid is attached, by an irreg- 

 ularly expanded calcareous base" (Sir Wyville Thomson). 

 There are five arms, and there does not appear to be any 

 anal opening. The genus Cyathidium, from the Chalk of 

 Faxoe, appears to be hardly separable generically from Holopus. 



We have, lastly, to consider very briefly the ordinary 

 sessile Crinoids, none of which are known to have appeared 

 in deposits older than the Secondary, and which have played 

 a comparatively unimportant part in geological history. The 

 first family of these is that of the Marsupitidce, in which the 

 calyx in many points of its structure resembles that of the 

 stalked Palaeocrinoids, but in which there is no peduncle. In 

 Marsupites (fig. 170), the " Tortoise-Encrinite," which we may 

 take as the type of the group, the calyx is of large size, and 



