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



side arm-plates, which belong to the first upper arm-plate, 

 are separated from it by reason of the development of the 

 radial shields and the position which it occupies, they main- 

 tain nevertheless their own relative position in respect to 

 the succeeding side arm-plates, and at the place where their 

 inner margins meet on the edge of the disk they mark the 

 middle of a side of the pentagonal body. In consequence of 

 the angle of direction at which the lateral plates are prolonged 

 from the median line, a large triangular interbrachial space 

 is enclosed between the first side arm-plates of two adjacent 

 radii and the central area of the disk ; this is occupied by a 

 single and uniform triangular plate, which is largest and 

 most conspicuous upon the dorsal surface. 



The extremity of each side arm-plate which enters into the 

 marginal edge of the pentagonal disk is notched, and bears 

 articulated thereon (normally) three short, stout, compressed 

 spinelets of uniform breadth throughout, their length being- 

 equal to or rather greater than the breadth of the side arm- 

 plates. These spines are truncate at the tip or slightly 

 rounded, and so closely placed that they join tightly up to 

 one another laterally, and form a continuous series, which 

 borders the entire margin of the disk. The breadth of the 

 fringe is rather greater at the middle of an interbrachial space 

 than at the extremities, in consequence of the gradual diminu- 

 tion in the length of the spines as they approach the angles. 

 Here and there, in the specimen under notice, the perfect 

 continuity of the series is somewhat interrupted, owing, pro- 

 bably, to accidental breakage and abrasion ; occasionally there 

 is an additional spinelet to a side arm-plate, whilst in other 

 cages two or three become merged together. Normally there 

 are about 48-50 spinelets in each interbrachial space. 



The manner in which the radial plates of the disk-series 

 become modified in the course of their passage onto the rudi- 

 mentary arms is very interesting ; and the transition into the 

 true side arm-plates of the little rudimentary ray takes place 

 so continuously and intelligibly that no doubt can exist as to 

 the homology of the strangely developed, band-like, lateral 

 plates in the disk with the true Ophiuroid elements of normal 

 form (PI. XX. fig. 3) . It will be observed that the two or 

 three terminal upper arm-plates, which fall within the limits 

 of the pentagonal disk, lose their rectangular shape, and that 

 the first change undergone by the oblong rectangular plate 

 normally consists of a lateral contraction by which the dis- 

 proportion of breadth to length is reduced ; this contraction in 

 the next outward plate is still more developed, its action 

 being greater on the proximal than on the distal margin of 



