METAZOA ECHINODERMA. 147 



of plates known as food grooves or ambulacra, and no 

 arms extended from the body. 



Another primitive form was Lichenocrinus dyeri Hall 

 (PI. 252, figs. 1-4). Little is positively known in regard 

 to this genus, and it is only on account of the structure 

 of the stem that a figure of it is placed on exhibition. 

 The portion preserved (figs, i, 2) was probably the 

 basal part which was attached to shells, etc., as seen in 

 fig. i. It was covered by irregular and imperforate 

 plates (fig. 2), which rested upon many radiating parti- 

 tions, seen in fig. 3, where the outer plates have been 

 weathered and have disappeared. They are also seen in 

 fig. 4, which is the lower or attached side. There is no 

 indication of arms or of areas of plates, the ambulacra, 

 but in the center a stem is visible, which probably sup- 

 ported the body. The genus is chiefly valuable in show- 

 ing the structure of a primitive stem. The five parts 

 making up the column can be distinctly seen (fig. 2), 

 whereas in the more specialized members of the group 

 they are consolidated so that their boundaries cannot be 

 made out. 



In the Cystoid Amygdalocystis (PI. 253, A, florealis 

 Billings), the ambulacra consist of a double row of imper- 

 forate plates and are concealed by covering plates. This 

 double row extended over the summit. The figure shows 

 several joints of the ambulacrum, each one of which bears 

 a pinnule ; also the body with many plates indefinitely 

 arranged, and the round stem. The mouth (m) and the 

 anus (a) are seen on the upper side. 



In Mesocystis ( = Mesites) (PI. 254, M. pusirefski), the 

 five ambulacra are present and are built on top of the 

 body plates. While the ambulacra are imperforate and 

 there are no holes between the plates, the body plates are 

 perforated. The position of the ambulacra in this genus 

 suggests their possible origin (see p. 149). The mouth 

 (PL 254, m) is at the summit and the anus (a) farther 

 down ; h is supposed to be a hydropore, though Lankester 

 thinks it is due to a boring parasite. 



