OF WESTERN SIND. 317 



Genus ECHINUS {Rondel), LinnS, 1758. 



1. Echinus subceenatus, Duncan & Sladen. Plate XLIX, Figs. 5 & 6. 



The test, which in its present state is considerably crushed, was probably much 

 depressed, and with the marginal contour subpentagonal. 



Ambulacral areas rather more than half the breadth of the interambulacral areas 

 at the ambitus, where their proportion is about five ninths of the latter, as the ambu- 

 lacral area measures 7*5 millim., and the interambulacral area 13*25 millim. at the 

 ambitus, in the specimen under notice. The ambulacral areas contract gradually as 

 they approach the apex. The poriferous zones are rather broad, with the pairs of pores 

 disposed in regular oblique triplets, the obliquity decreasing as the apex is approached. 

 The ambulacral plates are comparatively low and broad, their height at the ambitus 

 being about one third of the width. Each is composed of three poriferous plates, the 

 adoral and aboral ones being primaries, and the median a small wedge-formed demi 

 plate, which does not quite reach the middle of the compound ambulacral plate ; the 

 suture which bounds the adoral poriferous plate passes slightly upwards from this 

 point and obliquely, reaching the median ambulacral line near the middle of the 

 aboral facet of the inner end of the ambulacral plate. The ambulacral plates bear 

 one primary tubercle, placed close to the poriferous zone, and the series form a straight 

 and regular vertical line on each side of the interporiferous area, extending from the 

 apex to the peristome. The boss of the tubercle is of small elevation, but occupies 

 nearly the whole of the depth of the plate, and is surrounded by a faint scrobicule. 

 The mamelon is rather large and semiglobular, with scarcely any neck ; it is imperforate 

 and there is no crenulation. At the ambitus each plate bears a second tubercle nearly 

 as large as that just described and precisely similar, placed near the inner end of the 

 plate. Passing upwards towards the apex, these inner tubercles rapidly diminish in 

 size, and cannot be traced much further than midway between the ambitus and the 

 apex. Three or four tolerably large miliary granules, and frequently one or two 

 irregular small ones, are present on the plates. One of the larger ones is usually placed 

 near the aboral margin, and one on the adoral margin opposite the interspace between 

 the two large tubercles above noticed ; and frequently two small granules stand on the 

 aboral margin over the inner large tubercle. A miliary tubercle is in association with 

 the pore triplets, and is borne on the small median demi plate. 



The interambulacral plates are wide and low, their height at the ambitus is 

 one fourth of the breadth, the measurements being 1*75 millim. and 7 millim. re- 

 spectively. The actual measurement of height remains constant up to the apex, but 

 the breadth diminishes step by step on the abactinal surface until at the third coronal 

 plate from the apex the breadth and the height are nearly the same, viz. 1-75 millim. 

 The plates at the ambitus bear one primary tubercle, larger than the rest, and placed 

 slightly on the outer side of the middle line of the plate ; and this tubercle is the only 

 one whose series forms a continuous vertical row extending from the apex to the 

 peristome, the size being maintained throughout, and is a little larger than the primary 

 tubercles on the ambulacral plates, but similar in character. On the inner side of this 



