534 H. L. Haickina — The Species of Cidaris 



excavate, with a low parapet, which is in part rendered granular in 

 appearance by more or less deep crenehitions. There are about 

 eighteen crenelations on the platform. The mamelon is large and 

 prominent (diameter about 1 mm. less than that of the jiarapet), 

 circular, with an undercut neck and deep central perforation. The 

 scrobicular circle is complete, surmounted by a ring of from twelve to 

 fourteen (usually thirteen) secondary tubercles. Their mamelons are 

 about rS mm. apai't when their diameters are 1 mm. The diameter 

 of a secondary may be as much as half that of the primary mamelon. 

 The bosses of the tubercles are elliptical in shape, adjacent tubercles 

 being contiguous in tlie direction of their long axes. Outside the 

 scrobicular ring is a complete cj'cle of tertiaries, alternating regularly 

 with the secondaries. (In small examples the tertiaries are without 

 mamelons, but are always larger tlian the miliaries.) They are oval 

 in shape, the pointed part encroaching a little into the scrobicular 

 ring. They are usually less than half the size of the secondaries. 

 Outside the tertiaries thei'e appear, in very large specimens of 

 var. tnaxima, traces of an intermittent ring of quaternaries, but these 

 are not often distinguishable. Tbe miliaries are closely packed, and 

 diminish very little in size when traced to the margins of the plate. 

 There are usually no miliaries between the scrobicular ring and the 

 adradial and adoral margins of plates of var. typica. The granules are 

 circular in plan, and somewhat flattened. They are always without 

 any visible order of arrangement. 



In some of the largest specimens (var. maxima), when an adapical 

 plate may be as much as 27 mm. in transverse measurement, the 

 miliaries are proportionately more numerous than on average-sized 

 examples. 



An interesting abnormalitv in a plate from tliis region is shown 

 in PL XXV, Fig. 13. It will be noticed that the scrobicular ring 

 is double, the accessory circlet of secondaries being inside the 

 normal series and somewhat inclined towards the boss. There are 

 thirteen tubercles in the normal cycle, and sixteen in the inner, 

 accessory ring. 



Plates from near the ambitus are often almost twice as broad as 

 high, but otherwise differ hardly at all from those of the higher 

 cycles. The crenelation of the platform is usually less pronounced. 

 Both var. typica and var. maxima are as recognizable in this part of 

 the test as in the others. Measurements of an average ambital plate 

 of var. typica are : height 6*6, breadth 11 '4 mm. 



Towards the peristome the plates become almost square, so that 

 the interradial snture will have been nearly straight. In rare 

 cases the sci'obicule tends to become elliptical, but it is usually 

 circular. There are very few miliaries left on the plates near the 

 peristome, even in var. maxima. It is of interest to note that two or 

 three adoral interambulacrals are not frequently found in connexion. 

 This is probably due to a more complete union along the sutures due 

 to the pressure of the newly formed plates. The proximal adoral 

 plate may sliow A'arying stages of resorption, the inward bevelling 

 often commencing across a tubercle. The auricles are too much 

 waterworn in all the specimens available to be worthy of desci'iption. 



