CRINOIDEA. 



127 



veloped than the dorsal ; whilst the articular surfaces of the , 

 column-joints are usually morticed to one another by means of 

 crenated ridges which have a flower-like arrangement. Neither 

 of these latter characters, however, is constant. In the Meso- 

 zoic Rocks, also, appear for the first time remains of the 

 free Crinoids allied to the living Comatulce. The genus Sacco- 

 soma (fig. 77) is exclusively Oolitic, and appears to form in 

 some respects a connecting link between the true Crinoids and 

 the Ophiuroidea. In the Chalk the genus Comatula seems to 

 be represented ; and the Cretaceous genus Marsupites (the 

 " Tortoise Encrinite ") presents us with a form which is inter- 

 mediate between the fixed Crinoids and the forms which lead 

 a free existence when adult. 



Of the stalked Crinoids of the Mesozoic 

 period, the genera Encrinus, Pentacrinus, 

 Extracrinus, and Apiocrinns are especially 

 noticeable. In the genus Encrinus (fig. 

 80) the arms are composed of a double 

 series of alternating pieces, and carry pin- 

 nules on their inner surfaces. The joints 

 of the column are perforated by a small 

 round " alimentary canal." The best-known 

 species is the famous Lily-encrinite (E. lilii- 

 formis, fig. 80), which is characteristic of 

 the Muschelkalk, the middle member of 

 the Trias. 



In the genus Pentacrinus and the nearly 

 allied Extracrinus, the calyx is short and 

 composed of few plates (five basals and five 

 radials). The column is pentagonal, with 

 whorls of side-arms, and the articulating 

 surfaces of the joints are marked with pen- 

 tapetalous ridges. The arms are long, 

 slender, and branched. Amongst the best- 

 known species are the Pentacrinus fascicu- 

 losus of the Lias, and other nearly allied 

 forms of the same formation. 



In the Apiocrinidcz or " Pear-Encrinites " 

 (fig. 81), the animal was fixed to some 

 foreign object by a dilated base, and the 

 column was long and composed of numer- 

 ous joints. The articulating surfaces of the column-joints are 

 marked with simple ridges, and the stem is rounded and not 

 pentagonal. The last joints of the column gradually increase 

 in diameter, until they attain a size equal to the breadth of the 



Fig. 80. Encrinus lilii- 

 formis. Muschelkalk. 



