396 W. KEEPING ON THE DISCOVERY OF MELONITES IN BRITAIN. 



masses of various shapes, the larger ones remaining as broad bands 

 converging to the centre. Their upper surface is distinctly convex 

 both longitudinally and laterally ; on one border they are festooned 

 to receive the convex articulations of the adjoining interambulacral 

 plates ; but the opposite edge is a regular simply denticulated border 

 (o;, fig. 1). I therefore believe that these masses are broken halves 

 only of the ambulacral areas — the festooned edge being the outer 

 border of the area and the denticulated edge its middle line. An 

 examination of the constituent plates confirms this view ; for the 

 pores which they bear are all situated towards that edge of the plate 

 which is nearest the festooned border. 



We have seen that each half of an ambulacral area is convex 

 laterally ; they are in fact much more convex than the general sur- 

 face of a test 7 inches in diameter could have been ; and they must 

 therefore have stood out as ten broad ribs on the test, set in pairs 

 separated by a gentle valley in the median part of the ambulacral 

 areas, and by the broader planes of the interambulacral areas. 



The half ambulacral areas, as here preserved (see fig. 2), are com- 

 posed of six or seven ranges of small irregular plates, which are 

 broader than high, thicker than broad, and ornamented with minute 

 tubercles like the interambulacral plates. The middle ranges are 

 more regular. Each plate is perforated by a pair of pores, subtri- 

 angular where best preserved, and situated close to that edge of the 

 plate which is nearest the festooned border. Although the plates 

 are so irregular, yet the pores are most of them arranged in lines 

 and set in shallow grooves running longitudinally upon the areas. 

 On the internal surface of the test these grooves (or " poriferous 

 furrows," as they may be called) are represented by a correspond- 

 ing set of ridges. 



The tubercles form a very uniform granulation over the whole 

 test, but are best marked on the ambulacral areas. They are im- 

 perforate, without boss, and are of two orders : the larger ones, which 

 are most numerous, are surrounded by a smooth areola, bounded by 

 an elevated ring (fig. 5) ; contiguous rings are almost in contact. 



On the surface of one of the ambulacral masses and elsewhere on 

 the block some minute acicular spines are abundantly scattered ; the 

 microscope shows them to be sulcated, with a prominent collar at 

 the base (fig. 6). 



The dimensions of the specimen are as follows, viz. : — 



in. millim. 



Greatest length of specimen 7 \ 



Greatest breadth „ 7 



Average length of interambulacral plates .... 8 



Average breadth „ „ 6 



Average thickness „ „ 3-| 



Length of an ambulacral plate 4 



Breadth ,, „ 2 



Thickness „ „ 3| 



Breadth of half ambulacral area 19 



Length of spine 3-6 



