PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 11 



its diameter at the distal end. The distal corners of the basal plates are deeply cut 

 away to receive the radials. The crown is damaged on one side and one ray has been 

 broken away; it must be the right posterior ray, for there is no sign of the radianal 

 plate against any of the other radials. In three rays that are complete the arms are of 

 two, or sometimes three, brachials; they curl in above the apices of the orals. 



The surfaces of the lower part of the oral plates are only a little sunken below the 

 level of those of the basals, the radials, the costals (IBr t ), and the axillaries with which 

 their edges are in contact. The texture of all the plates is coarse but that of the orals 

 coarser than any; the surface of the proximal part of each is deeply pitted and some of 

 the pits carry pale yellow spherical bodies which must be the glandular sacs known from 

 the posterior end of the embryo. In preparations of embryos slightly older than that 

 figured by Mortensen the sacs lie close against the oral plates, and, although they are 

 not partly enclosed in their stereom, they might easily become so as growth proceeds. 

 Such sacs occur in all the older larvae of this series. 



The fourth larva has a crown 2.1 mm. long and a column 7 mm. long. The stem 

 is of 41 columnals and a thick lobed terminal plate which appears to be simple. There 

 are 10 very short discoidal proximal columnals, those nearer the basal cup wider than 

 the others. Most of the remaining columnals in the first half of the stem are nearly as 

 long as broad but those of the distal half become progressively and gradually shorter 

 towards the terminal plate. 



The basal cup has strongly rounded sides. The radials are widely separated from 

 one another. The arms of only two rays are complete; they are of five brachials. 

 The radianal plate is in broad contact with the right posterior radial which makes it 

 asymmetrical; it is in contact with the proximal half of the left posterior radial. It 

 overrides the oral; no naked perisome is to be seen in this interradius. In other inter- 

 radii the orals may be separated from the basals by a narrow strip of perisome. 



The edges of the orals become free opposite the costals, beyond which the plates 

 narrow as they turn in over the disk. 



There are side plates along the ambulacra of the arms. 



The ninth specimen in range of size has the crown 3 mm. long. The column is 

 incomplete, only 29 columnals remaining. The first is short and closely associated 

 with the basal cup. The second to fifth are short and discoidal. The sixth is dis- 

 coidal but longer. The seventh is about half as long as broad. The eighth to near 

 the twentieth are as long as broad, the remainder slightly broader than long. 



The crown is similar to, though larger than, those of the previous stage described. 

 Each of the basal plates is swollen so that the sutures between them run along depres- 

 sions. The distal half of the radianal plate lies, not on the oral, but on a lobe of tissue 

 at a higher level than, and overlapping the proximal edge of, the oral plate. This is 

 taken to be the beginnings of the anal tube. 



As in a slightly smaller stage from the same station there are glandular sacs on 

 other ossicles than the orals. 



The arms are of seven or more brachials. 



The fifteenth specimen has the crown 4 mm. long and a column 10.5 mm. with 

 39 columnals and a thick slightly lobed terminal plate. The first columnal is incom- 

 plete, so that the second, although for the most part in touch with it, is in touch with 

 the basal cup too. The second to seventh columnals are short, gradually increasing 



