168 Bulletin Vanderbilt Marine Museum, Vol. VII 



larger, the walls narrowed, rounded, the ridge distinct, rounded. 



The interambulacra have the areoles of medium size and 

 depth, well separated, except when the proximal two are con- 

 tiguous, those of the actinal area variably transversely oval. The 

 two upper areoles in one or both interambulacral series are nor- 

 mally rudimentary. The tubercle is small, with a low boss, the 

 crenulation faint or nearly obsolete in nearly all specimens except 

 two young forms, in which it is distinct. The margin of the areoles 

 is only very moderately elevated ; the scrobicular tubercles are of 

 moderate size. The entire plate surface outside of the scrobicular 

 ring bears numerous, closely-set, secondary tubercles, which de- 

 crease serially in size toward the median line, which is usually 

 only slightly sunken, there normally being no bare admedian or 

 adradial area. The median space is from one-half to two-thirds 

 of the width of an areole. 



The apical system, which is of somewhat smaller proportion 

 in very large specimens, averages from a little less to a little more 

 than half of the horizontal diameter in specimens of moderate 

 size. All of the plates are regularly paved with close-set, small, 

 subequal tubercles, except around the usually bare margin. In 

 the present series the ocular plates are all exsert. Jackson (1912) 

 recorded specimens in which these ocular plates were all nar- 

 rowly insert. The genital pores are small, nearer to the center 

 than to the outer margin of the plates. The periproct consists of 

 an average number of plates. 



The primary spines are from 1.4 to 1.6 times the horizontal 

 diameter of the test in medium or large specimens, and sometimes 

 twice this diameter in young forms. These primaries attain their 

 greatest diameter about one-fifth of the length from the base ; the 

 diameter varies from 2.4 to 3.1 millimeters; the spines are either 

 slenderly fusiform in varying degree or distinctly cylindrical, 

 variously tapered to an obtuse apex or slightly widened apically. 

 The greater portion of the primaries of the present series of 

 abyssicola are of the slender, cylindrical style, tapered distally to 

 an obtuse apex. The shaft appears smooth, white, with fugitive 

 rosy tints; under magnification it is seen to be longitudinally 

 striated by 18 to 24 ridges, each composed of series of low warts 

 or granules which tend to become obsolete on the larger spines. 

 These granules on the smaller oral primaries tend to form more 

 distinct and relatively longer serrations proximally, while distally 



