a new Genus of Echinodermata. 407 



probability, entirely retractile. Each pore is protected by 

 tentacle-scales : one, which is large and lanceolate, is situated 

 on the interradial side of the aperture, and is very frequently 

 found reflected or bent quite backward towards the actino- 

 storae ; another scale, smaller and more rotund, occurs on the 

 side of the pore adjacent to the under arm-plate, but is often 

 wanting on the pores of the outer portion of the radii. Both 

 of these are unmistakably " scales " in the proper sense of 

 the term, and bear no resemblance whatever to the usual 

 form of tentacle-" papillas " found in many Ophiurids. In 

 addition to those just described there is frequently a free 

 ovoid-shaped scale present on the outer or aboral portion of 

 the membrane of the tentacular aperture. 



The last two or three arm-segments included within the 

 disk are worthy of notice, since they show the gradual stages 

 of modification which the radial series undergo in the course 

 of their passage onto the rudimentary arm. In the last 

 joint but two the breadth of the under arm-plate is very greatly 

 reduced, the proportion to length being less than one halt", 

 and the aboral end forms, at the same time, two sloping- 

 facets, in consequence of the encroachment of the succeeding 

 pair of side arm-plates, which extend, by the prolongation ot 

 their adoral margin, quite up to the median plate, the tentacle- 

 pore being scooped out of the outer margin of the plate itself, 

 as usual in Ophiuridge. In the last under arm-plate but one 

 the diminution in size becomes still greater, whilst in the 

 segment which follows the side arm-plates meet for more than 

 half their length in the median line, the under arm-plate 

 being reduced to a small subtriangular plate enclosed in a 

 notch on their outer margin ; this is the basement joint of the 

 rudimentary arm, and is situated half within and half without 

 the pentagonal disk ; the side arm-plates belonging to this 

 segment bear three marginal spines, which are only very 

 slightly modified from those of the marginal fringe. Passing- 

 outward to the second joint of the free ray, the under arm- 

 plate is very insignificant, and the arm-spines are present as 

 short, stout, rounded spinelets of true Ophiuroid type, whilst 

 in the third joint the under arm-plate has become quite micro- 

 scopic and the spines reduced to mere papillaj (PL XX. 

 figs. 6, 6 a). 



Concurrent with the changes noted above in the plates of 

 the terminal segments of the disk, the pore-areas become very 

 greatly reduced in breadth towards the extremities of the 

 radii ; for whilst midway between the centre of the actinostome 

 and the angle of the disk the half pore-area on either side is 

 broader than the median range of under arm-plates, the last 



