anew Genus of Echinodermata. 405 



the plate. This gives to the side arm-plates the appearance 

 of resting upon the preceding upper arm-plate, at the same 

 time inducing therein a pentagonal figure. 



In the last radial segment included within the pentagonal 

 disk, the upper arm-plate has generally been so much reduced 

 in size that the side arm-plates meet and hold it within the 

 angle which they form, whilst the upper arm-plates, which 

 follow upon the rudimentary ray, have become subtriangular 

 or fan-shaped and quite insignificant in size. 



A slight variation, consequent on the irregularity of some 

 of the plates, is noticeable in one of the radii of the present 

 specimen (PI. XX. fig. 4). This consists of the radial series 

 apparently passing off into the free rudimentary ray before 

 the margin of the disk is reached — a deceptive effect arising 

 from a modification of the last segment included in the disk, 

 by which the side arm-plates and their marginal spinelets 

 remain isolated and form a continuous collar-like rim beneath 

 the ray connecting the lateral fringes of the two neighbouring- 

 sides, whilst in all the other rays the series of marginal spines 

 is separated by the presence of the free armlet. 



In one of the rays considerable irregularity occurs in con- 

 sequence of the splitting-up of some of the upper arm-plates 

 and the coalescence of two or more of the side arm-plates, 

 the former circumstance being, to a certain degree, sug- 

 gestive of the broken character of these plates in Ophioplocus 

 and some Ophiurce. 



Actinal Aspect. — The mouth is situated in the centre of the 

 under or actinal surface, and is surrounded by ten large plates 

 arranged in pairs, representing side mouth-shields. They are 

 subtriangular or somewhat coulter-shaped, with the small end 

 directed inward ; and the two plates of a pair touch one 

 another along their interior half only, the outer portion 

 arching outward towards the radii. These mouth-plates pro- 

 ject prominently towards the centre of the actinostome, the 

 mouth-slits being consequently deep and wide; and each 

 mouth-plate (pseudo-side mouth-shield) bears on its outer 

 side three or four short, oblong, flattened mouth-papillse ; and 

 a single, large, semicircular papilla stands at the apex of the 

 pair of mouth-plates (PI. XX. fig. 5). 



A mouth-shield is present in only one of the interbrachial 

 spaces. It is roundly shield-shaped, and is, in all probability, 

 the representative of the madreporiform body — an inference 

 drawn from the fact that this plate bears a raised tubercle 

 punctured with a large pore near one of its outer corners. 

 No jaws (scutella oralia), no jaw-plates {tori angular es) f and 

 no teeth are present. 



