OF WESTERN SIND. 845 



on the flank of the test. The three lateral swellings, of which the middle one is the 

 least developed, are symmetrically placed on both sides of the test, and although they 

 do not appear to relate to any structural peculiarity on the dorsum, they are, on the 

 actinal surface, the points whence as many series of concentric lines of small tubercles 

 arise, giving a very varied appearance to the ornamentation of the base. 



The actinal surface of the test is not flat, for there is a slight hoUowness around 

 the mouth, a gentle convexity of the anterior interradia from before backwards, a less 

 convexity of the lateral interradia from side to side, and a decided carination along the 

 median line of the sternum, culminating far back in a pointed downward-projecting 

 summit. This last is crossed by the subanal fasciole and forms the spot on which 

 the test rests posteriorly. Since the base rests on this projection and on the convexity 

 in front of the peristome, the greater part of the actinal surface does not touch the 

 supporting substance, and there is a decided space seen especially between the peri- 

 stome and the surface on which the Urchin rests. The posterior lip of the peristome 

 projects downwards, is semi-nodular, and carries many large granules. 



Behind the posterior projection of the keel and within the subanal fasciole, the 

 test slopes upwards and backwards, to reach the rim of the broad smooth surface, which 

 is below the depression for the anus. • 



The lower part of the truncation, or that which is within the semicircular area 

 enclosed by the subanal fasciole, is convex from side to side and projects along the 

 median line as a faint ridge. The smooth surface above the upper part of the fasciole 

 is very concave and at the upper part of this depression is the periproct. Its normal 

 shape is pyriform, the broad base being towards the actinal part of the test, and the narrow 

 and angular upper part being close below the edge of the low rounded broad keel of 

 the odd interradium. In that sense the periproct is marginal ; but it is only near, and 

 not in or on, the upper and posterior margin of the odd interradium. It is by no means 

 marginal according to the usual application of the term, for it is some distance above 

 the posterior margin, even if that part is supposed to be the top of the subanal 

 fasciole. The posterior edge of the odd interradium forms a broad and low arch, and 

 where the point of the periproct comes near it the edge is thin ; but on either side of 

 this central spot there is a thickening of the sides of the arch. The posterior structures 

 give a greater vertical thickness to the end of the odd interradium than is seen in the 

 anterior part of the test. 



The sulcus for the anterior ambulacrum is well developed as far back as a line a 

 little in advance of the anterior poriferous zones of the antero-lateral ambulacra, where 

 it becomes shallower but broader ; thence it diminishes to the apical system. This is 

 usually in a continuation of the shallow groove, which may even pass as far back as the 

 posterior angle of the internal fasciole. 



The apical system is small, narrow, broader behind than in front, and long, the 

 length being due to the backward projection of the madreporite, which passes as a 

 triangular structure between the highest plates of the odd interradium (5) and 

 separates plates 1 and 2 of zone " a " from their fellows of zone " b," on the other side 



3 a 



