212 DISCOVERY REPORTS 



definitely no suboral spine ; hepatic diverticula visible through transparent peristome. 



Distance from mouth-plates to end of ray, 1-25 mm. 



Stage 4. Diameter 5 mm.; diameter of peristome 17 mm.; distance from mouth- 

 plates to end of ray i -6-1 7 mm. The disk is now only slightly convex ; the disk plates are 

 subcircular (Fig. G, 3) and as well marked as in stage 5 ; the spinelets (^a-b) end in 3 or 

 4 short thorns. On the rays the rudiments of the first arch have appeared: 3 dorsal 

 plates, the median or radial with a spinelet and a circle of 6-10 pedicellariae ; the laterals 

 (superomarginals) with the bud of a spine ; next to the adambulacrals on either side is 

 the rudiment of an inferomarginal plate. The 3 adambulacral spinelets are now much 

 more conspicuous and may carry several pedicellariae. The mouth-plates are larger, 

 with a short suboral spine, and the second shorter marginal spine adjacent to median 

 suture. The terminal ruff of pedicellariae is still larger and there are 8 slender con- 

 spicuous terminal spines. The outline of the stomach and hepatic diverticula is clear 

 through the transparent peristome but the mouth and lips seem to be covered by tissue. 



Staoe 5 (PI. XVI, figs. 10, 1 1). Diameter 6-6-2 mm. ; diameter of peristome, 2-5 mm. ; 

 distan^'ce from mouth-plates to tip of ray, 1-85 mm. This differs from the last chiefly in 

 increase in size. The rays are slightly longer and there are 6 pairs of tube-feet and 4 

 prominent adambulacral spines, but no additional oral spines nor abactinal ray spines, 

 and no spine on the rudimentary inferomarginal plate. This is the stage shown in 

 photographs, and it seems probable that the young soon leave the nursery through the 

 ventral side of the brood "basket". 



Growth stages, (i) The youngest post-nursery stage is a 12-rayed specimen from 

 St. 123 having the following dimensions: diameter 14 mm., R 6-7 mm., r 2-5 mm. 



The disk is not convex and is paved with relatively large, roundish, thin, over- 

 lapping plates bearing each a 3-pronged spine. This plating ceases abruptly at base of 

 ray, to be followed after a very narrow interval of skin by the first skeletal arch composed 

 of 7 plates, of which the unpaired mid-dorsal is usually 3-lobed, larger than the others, 

 and carries a prominent sacculated acicular spine; the other shorter spines without a 

 sacculus. The second arch, well spaced from the first, has a tiny mid-dorsal spine and 3 

 conspicuous sacculated laterals. Then there are two widely spaced arches with 2 lateral 

 sacculated spines, and rudimentary dorsal plates ; and finally a rudimentary fifth arch 

 near the conspicuous terminal sacculus. 



The adambulacral and mouth spines are relative to length of ray much longer than in 

 adult, and the former bear a lateral sacculus of pedicellariae. The second marginal 

 mouth spine to be developed is now longer than the lateral. The first pair of adambula- 

 crals only touch by their inner ends adjacent to oral plates, and the first and following 

 interadambulacral joints show no sign of a syzygy. These proximal adambulacrals are 

 more elongated than in the adult. This condition and the separation of the first pair of 

 adambulacrals at oral angle is similar to the structure of Brishigella. 



(2) The next larger specimen (St. 39) is somewhat mangled but two of the 13 rays 

 can be measured : diameter 24 mm., R 1 1-12 mm. r 375 mm. There are now 7 skeletal 

 arches. The first 3 are clearly complete. The fourth to sixth are technically completed by 



