from the Loxoer Greensand. 535 



Amhulacral Plates. — There are, on the average, twenty-four simple 

 primaries to each interambulacral. TJie pore-pairs are almost hori- 

 zontal at the adapical end of the ambulacra, in deeply sunken 

 poriferous zones, and become slightly inclined towards the perradial 

 suture for most of their course. The pores are practically circular, 

 and encircled by a faint peripodium. The pores of each pair are 

 separated by a prominent granule of rather greater diameter than 

 the pores themselves. Along the perradial part of the ambulacra 

 high ridges of prominent granules, one granule to a plate, pass the 

 entire length of the areas, and, except in very large specimens of 

 var. maxima, occupy the extreme perradiad portions of the plates. In 

 these large examples, near the ambitus, traces of irregular and quite 

 minute granules occur, but these never reach half the size of the 

 granules of the main series. The horizontal measurement of one 

 amhulacral plate is about 2 mm., of which 1"2 mm. are occupied by 

 the poriferous zone. Thus the total width of the ambulacrum is 

 about 4 mm. — roughly one-sixth that of the interambulacrura. 



Primary Radioles. — These radicles vary considerably in size, the 

 largest obtained being 61 mm. in length and 4'25 mm. in breadth 

 (some fragments indicate even larger size), while an average 

 specimen gives corresponding measurements of 40 and 4 mm. The 

 stem is cylindrical, never appreciably inflated, and covered with 

 regular granular ridges. The space between the ridges is finely 

 shagreened. The distal end of the stem is often slightly expanded, 

 and alternate ridges, becoming carinate, develop into a stellate 

 corolla of small size. The collerette is very long (the proportion 

 varying but sliglitly with the length of the radiole), the average 

 measurement being about 10 mm. Its diameter is very slightly less 

 than that of the stem, and its surface is smooth. The ' bourrelet ' 

 which separates it from the stem is well marked, cylindrical, and 

 almost at right angles to the axis of the radiole. The annulus is 

 apparently smooth and not prominent. The base is inflated to about 

 1"5 times the diameter of the collerette, but the acetabular ring 

 (which is often crenelated) forms a circle of diameter equal to that 

 of the bourrelet. The acetabulum is deep, with a wide central 

 perforation. Some radioles show a diminution in the diameter of the 

 collerette near the bourrelet, making the proximal end of the radiole 

 broader than the rest. 



There are three or four large radioles in Mr. Treacher's collection 

 which are smooth and tapering (PI. XXV, Fig. 14^). I regard these 

 as variants of C. faring donmsis. I have a specimen of C. scuptrifera, 

 Mant., from the Upper Chalk, which has attached to it many normal 

 denticulate radioles and several devoid of ornament ; the smooth 

 radioles from Faringdon seem analogous. They can be easily 

 distinguished from radioles of P&eudodiadema, with which they are 

 associated, by their greater proportionate breadth and general 

 size. Some intermediate stages show faint traces of the typical 

 faring donensis granulation (cf. PL XXV, Fig. \Aa). 



Secofidary Radioles. — I ascribe the larger secondary Cidaris radioles 

 from Faringdon to this species. They are small and flattened, 

 averaging 4 mm. in length and 1 mm. in breadth at the base. Their 



