or WESTEEN SIND. 141 



r^ of the width of the interporiferous area, and thence the width of the poriferous 

 zone decreases gradually to the margin. The widest part of the interporiferous area is 

 at the margin, where it is slightly wider than twice the width of the poriferous zone at 

 its widest part. The petaloid portion of the poriferous zone passes gradually and almost 

 imperceptibly into the extrapetalous part, the pairs of pores being continuous throughout. 

 In the extrapetalous part of the area both pores are small and round, and separated 

 only by a very narrow dissepiment ; and the pairs are placed oblique, their distance apart 

 and the angle of obliquity to the horizontal increasing as they approach the peristome. 



The structure of the ambulacra is interesting, for, notwithstanding the strict uni- 

 serial character of the poriferous zone, the poriferous plates are distinctly divisible into 

 triplets, this arrangement being regular and invariable in the petaloid and in the extra- 

 petaloid parts of the zone alike. Throughout the whole zone a smaller plate is inter- 

 calated between two larger plates, and of these the adoral is always slightly the largest. 

 The small intercalated plate. is only just large enough for its pair of pores, and scarcely 

 extends into the interporiferous area. In the petaloid part of the zone the small plate 

 is long and narrow (fig. 6), and in the extrapetaloid part it is almost equilaterally sub- 

 triangular (fig. 7). On the actinal portion of the test the large poriferous plates are 

 much deeper (^. e. longer), and consequently the pairs of pores are wider apart (fig. 8). 

 In the petaloid portion of the zone the inner pore is marginal, indenting the adoral edge of 

 its plate ; in the extrapetalous portion both pores are within their plate, and equidistant 

 from the ad- and aboral margins. Reference to the figures of these diff"erent paxts of 

 the poriferous zone will give a better idea of the arrangement of the plates and pores 

 than a lengthy verbal description. 



The ornamentation of the interradial portions of the test consists of a great number 

 of small, uniform, equidistantly spaced primary tubercles, faintly crenulated, and with 

 a small perforated mamelon, placed in a shallow scrobicule, surrounded by a rim of 

 small, uniform, distinct miliary granules. Occasionally the single ring of granules which 

 separates neighbouring scrobicules is augmented by additional granules, and a few extra 

 ones fill up the interspaces. At the margin the primary tubercles are rather more 

 crowded, and on the actinal surface they are a shade larger. The ornamentation of the 

 interporiferous areas is precisely similar to that of the interradia. 



The peristome is slightly excentric in front, and placed in a well-developed 

 depression of the test ; it is large, subpentagonally oblong in form, and oblique 

 in position, the longer axis being directed towards the right anterior interradium. 



The periproct is very large, and occupies more than half the area between the 

 peristome and the margin ; it is longitudinally elongate and subpyriform, and its length 

 is fully one third more than the greater diameter of the peristome. The contracted 

 portion of the pyriform aperture is directed towards the peristome, and the distance of 

 this extremity from the peristome is usually less than that which intervenes between 

 the aboral margin of the periproct and the margin of the test. 



Variaiions. The chief modifications, distinct from those dependent on growth, which 

 may be noticed in the material at our disposal are as follows :■ — (1) a slight increase in 

 the relative proportion of length to breadth ; (2) a greater uniformity in the curvature 



U 



