CH^TODIADEMA PALLIDUM. 125 







is at once distinguished from granulatum by the comparatively large size of 

 the abactinal system (PI. 59, fig. ^, compare with that of Ch. granulatum, 

 PI. 1, fig. i), which is much smaller in proportion to the size of the test. 

 In the specimen of pallidum, figured in PI. 59, figs. 3, 1^, the abactinal sys- 

 tem measures 19.50 mm., the anal system 11 mm., and the actinal system 

 only 10.25 mm. in diameter. In a larger specimen (PI. 59, fig. ^) the 

 abactinal system measures 22 mm. There are no papillae covering the 

 genital openings, which are surrounded by a slightly raised ring (Pis. 56, 

 figs. 2, 5 ; 59, fig. 4.). The oculars and genitals are all in contact with the 

 distal ring of large irregularly pentagonal anal plates, inside of which are 

 two rings of irregularly polygonal smaller plates, and immediately at the 

 base of the anal tube the anal membrane is covered with minute papilla?. 

 The large anal plates carry small miliaries and an occasional secondary 

 tubercle. The genitals as well as the oculars are sparsely covered with 

 miliaries (PI. 56, fig. S). The madreporite covers the greater part of the 

 madreporic genital (PI. 56, fig. 5). 



The actinal system is markedly pentagonal, with prominent actinal 

 indentations. The five pairs of buccal plates form a closed ring round the 

 mouth, and outside of them the actinal membrane is covered with a narrow 

 belt of small more or less elliptical plates. At the actinal angle of the ambu- 

 lacra a larger plate is found (PI. 56, fig. l). The actinal system is deepl}^ 

 sunken (PI. 59, fig. 1) far more than is the case \n granulatum, which, judging 

 from the figure (PI. 1, fig. 3) given by Mortensen, is comparatively flat. 

 The actinal part of the ambulacra is most indistinct in granulatum (Mortens. 

 PI. 1, fig. 3), while in Ch. jMlliduni (PI. 59, fig. 3) the pores form well-marked 

 zones. The close tuberculation of the actinal surface is well shown in 

 PI. 59, fig. 3, and more in detail in PL 56, fig. 1. It will be seen that 

 the vertical rows of primary tubercles, diminishing in size, extend almost to 

 the actinal system, and that the close granulation of the actinal surface 

 mentioned by Mortensen in the interambulacral area is limited to the 

 interambulacral space of that surface and does not cover the whole actinal 

 surface nearly to the ambitus, as is the case in granulatum. It is true 

 that the large tubercles of the actinal surface of pallidwn, which form the 

 continuation of the vertical columns of primary tubercles from above the 

 ambitus, are different from those at the ambitus and above it. The latter are 

 perforate and crenulate, while the former are not usually perforate and are not 

 crenulate (PI. 56, fig. 5). It is quite possible in our species to distinguish 



