Boo7ie, Echinodermata, Cruises of "Ara" and "Alva" 187 



tenth of the ambulacrum, posterior to which point the pedicels 

 become closely crowded together, continuing for three or four 

 pairs in alternately placed dual series, then abruptly widening to 

 four pedicels to each transverse series, these placed in close-set 

 alternate arrangement for about eight series of four pedicels 

 each, these being succeeded by six pedicels per transverse series, 

 which arrangement persists for the next three-eighths of the 

 length of the ambulacra, thence decreasing to four pedicels per 

 transverse series, for the space of five or six rows, these being 

 followed by an alternate placed dual series of twelve to fourteen 

 pairs, those pedicels toward the aboral end being less distinct, 

 more widely separated, the most posterior pair being a little dis- 

 tance (about 5 millimeters) from the aboral aperture. The median 

 ventral ambulacrum of the trivium is like the submedian pair of 

 ambulacra, but has the pedicels persist in transverse series of four 

 per each row nearer to the oral end than do those of the lateral 

 pairs ; there being only eight pairs of dual series of pedicels and 

 these placed closer together in more nearly regular series on the 

 ventral ambulacrum. Posterior to these eight pairs in dual series, 

 the arrangement of four pedicels per transverse series persists 

 for only six or seven rows, these being replaced by the fairly large, 

 closely crowded pedicels, in transverse series of six alternately 

 placed pedicels, which continues for nearly half the length of the 

 ambulacra and is followed posteriorly by about eighteen rows com- 

 posed of four pedicels each, in rather close fonnation, these 

 quadri-series of pedicels extending much further posteriorly than 

 do those of the outer pair of ventral ambulacra, there being only 

 five or six dual series in the median ambulacra posteriorly, the 

 last pair of these pedicels being just a little outside of the aboral 

 area, about in line with the most posterior pairs of the outer ven- 

 tral ambulacra. 



The subdorsal ambulacra, forming the bivium, have the pedi- 

 cels rudimentary, papillae-like, fewer and more scattered, in 

 approximate dual series, the papillae of a pair being widely spaced 

 from one another as well as from adjacent pairs. Toward the 

 median area of each ambulacrum, there is for a very brief space 

 a few secondary, small papillae in addition to the dual series, but 

 at no place do these attain the four per transverse series arrange- 

 ment of the trivium ambulacra. 



