MONOGRAPH OF THE EXISTING CKINOIDS. 97 



Perisome. The noncalcareous integument covering the ventral surface of the 

 animal; in general this term is restricted so that it refers only to the integu- 

 ment covering the ventral and lateral portions of the disk (see figs. 1, p. 60, 

 2, p. 61, 6, p. 63, and 15-18, p. 67). 



Perisomic interradials . Perisomic plates arising secondarily between the division 

 series on the outer (dorsal) surface of the disk (see fig. 104, p. 1G7, and p. 339). 



Perisomic plates. More or less irregular plates developed within the eutis (sec figs. 

 8, p. 63, IS, 19, p. 67, and 122, p. 191, and p. 195). 



Perisomic skeleton. The dermal skeleton developed in the perisome of the adult 

 animal. 



Perisomic spicules. Spiculcs developed within the cutis. 



Peristome. The depressed area on the disk immediately surrounding the mouth 

 (see fig. 15, p. 67). 



Perradial. Same as radial, as contrasted with interradial. 



Phytocrinoid. See Pentacrinoid. 



Pinnulars. The segments of the pinnules (see figs. (5-S, p. 63). 



Pinnulation. The arrangement of the pinnules (see pp. 112-113). 



Pinnule ambulacra. The ambulacral grooves on the ventral surface of the pin- 

 nules, in contrast to those on the ventral surface of the arms and of the disk. 



Pinnule sockets. The articular facets on the brachials to which the pinnules are 

 articulated; they are in origin degenerate muscular fossae (see fig. 32, p. 71, 

 and p. 273). 



Pinnules. The slender jointed structures which border the arms (see figs. 1, p. GO, 

 2, p. 61, 3, p. 62, 6, 8, p. 63, and pp. 112-113). 



plate. As usually employed this term covers calcareous structures \vhu-h are much 

 broader than their interior-exterior diameter. 



Plated ambulacra. Ambulacra which are protected by well-developed side and 

 covering plates (see figs. 7, p. 63, 18, 19, p. 67, and 55, p. 81, and p. 112). 



Plated disk. A disk upon which secondary calcareous dermal plates are developed 

 (see figs. 7, p. 63, and 55, p. 81, and pp. 111-112). 



Polar cirri.- See Small mature cirri. 



Posterior arm. See Axis Ib and Orientation 3. 



Posterior interradial area. See Axis and Orientation. 



Posterior radii. The radii on either side of the anal interambulacral area (see 

 figs. 22-25, p. 69, 117, p. 183, and pp. Ill, 152-101). 



In certain of the Comasteridse the left posterior radius becomes curiously 

 modified, and is then often distinguished simply as the posterior radius, the right 

 posterior radius being considered in this case as an anterior radius (see figs. 

 27, 28, p. 69, and pp. Ill, 152-161). 



Posterior ray. In the Comasteridre (see Orientation 3). 



Posterior rays. See Posterior radii 



Post-palmars (IVBr, VBr, VIBr, etc.) Series of two, three, or four ossicles, 

 always ending in an axillary, developed beyond the palmars (IIIBr series). 



When this term is used the successive series are designated as first post- 

 palmers (IVBr series), second post-palmars (VBr series), third post-palmars 

 (VIBr series), etc. 



