410 Mr. W. P. Sladen on the Structure of 



animal, to span across the ambulacral area at each segment. 

 It would seem also that there is occasionally a thickening of 

 the side arm-plate in order to form a junction with, or pro- 

 longation of, this plate ; but whether such a development 

 is the rule in every segment, I am unable to say, without 

 destroying the specimen. It may be noted that this thicken- 

 ing of two neighbouring side arm-plates is productive of a 

 deep trench or groove between the side arm-plates, following 

 the direction of their line of suture and leading into the main 

 interradial cavity. Close up to the abactinal wall is the trace of 

 a thin, delicate, arched plate, which forms the boundary of 

 the tentacular cavity (fig. 8,f) : the extraordinary space occu- 

 pied by the tentacular pit, to the exclusion (almost absolutely, 

 I believe) of the muscular area, is highly remarkable. This 

 is rendered very striking when a comparison is made of dia- 

 grams of the respective parts in Astrophiura and a typical 

 Ophiuran. 



The internal skeleton of the aborted ray-extensions is quite 

 rudimentary and abnormal, the arm-plates being increased to 

 such a thickness as to form almost solid joints, and the com- 

 munications between the succeeding segments being of the most 

 limited character. This is apparent in the first free segment 

 of radius iv (fig. 9) ; whilst a little further out upon the ray, 

 in the fourth segment of radius v, the axis has the appear- 

 ance of a miniature vertebra located in the centre of a dispro- 

 portionately thick and solid arm-joint (fig. 10). 



Skin-appendages. — The plates of the entire test are smooth 

 and naked, there being no trace of pedicellariee, granules, 

 spinous stumps, or any corresponding developments what- 

 ever. 



Locality, Preservation, and Colour. — The present example 

 of this Echinoderm was taken at Madagascar, and sent over 

 by a collecting agent along with a number of littoral or shal- 

 low-water Asteroids and Echinoids from the same locality. 

 The specimen was simply dried ; and its colour in that condi- 

 tion is yellowish white, the marginal fringe of spinelets ap- 

 proaching a light brownish shade, and the actinal or under- 

 side a deeper tint of the same. 



Measurements of an Individual. 



niilliin. 



Diameter of pentagonal body 14 



Length of side of „ 975 



Length of a radius 8 



Diameter of the centro-dorsal " ophiuroid " disk. . 8 



Radial shields, length to breadth 2 : 1*5 



Second upper arm-plate, length to breadth 0*5 : 1*3 



