Studies on tJte Echinoidea Holectypoida. 217 



structures, whether radial or interradial. Only the posterior part 

 of the periproct (Fig. 1, 3) is exposed ; this is almost vertical in 

 situation (owing to the inflation of the test), and extends with sub- 

 parallel sides to about the anibital level, there closing rapidly (with 

 slight asymmetry) to an obtuse angle. The exposed portion is 

 definitely exocyclic. 



The ambulacra (Fig. 5) are extremely narrow, straight, faintly 

 convex transversely, and less prominent than the interradial 

 carinae. The plating is entirely piimary in visible parts, but may 

 have been more complex towards the peristome, where a slight 

 increase in width suggests development of a small hypophyllode (see 

 Fig. 2). The small plates are perforated by very minute pore-pairs 

 near to their adradial margins ; each bears a granule perradially, 

 and many support two, when the inner (perradiad) granule is very 

 insignificant in size. The outer (main) granules are subequal, but 

 those on every third plate are j^erhaps a little larger than those 

 intervening. There are about ten ambulacrals to each inter- 

 am bulacral plate on the ambitus. 



The interambulacra are very broad (Figs. 4, 6), and obtusely 

 carinate at a distance (on the ambitus) of 3-5 mm. from the adradial 

 sutures ; they are slightly tumid along the interradial lines, 

 except in the case of 5, which is markedly concave in that region. 

 The plates are low and broad, especially on the ad apical surface ; 

 their transverse margins are deflected adorally in crossmg the 

 carinae ; those of area 5 are sharply bent adorally near the 

 periproct (see Fig. 3). The ornament is profuse ; a single seiies 

 of primary, scrobiculate, ? perforate tubercules is placed on each 

 carina ; these tubercules are fairly large (diam. of scrobicule 

 1 mm.) on the adoral surface, but become suddenly reduced on the 

 anibitus (and above) to a diameter of only -5 mm., perhaps in- 

 creasing again near the apex. The rest of the plate-surface is covered 

 with a granulation approximately homogeneous, but including 

 occasional scrobiculate granules larger than the rest. No 

 determinable order appears in the arrangement of the minor 

 ornament. 



Note. — Both sj)ecimens are jjreserved in a pale grey limestone 

 of great hardness and almost lithographic texture. In the holotype 

 the greater part of the test is wanting ; the remaining fragments 

 are of a yellowish colour, and display the ornament in a much 

 weathered condition. Part of amb. I, round about the ambitus, 

 shows the sutures \^ery clearly. Much of the adapical surface is 

 obscure, partly by fracture, and jiartly by incrustation. The region 

 of the " posterior surface " around the periproct seems to have been 

 scraped with a knife, but very little thickness of test has been 

 removed. The peristome is blocked by a shell, and, since the test 

 is missing there, there is some difficulty in determining its true 

 outline. The paratype has been carved out of its matrix, and 

 also broken into three pieces. It is slightly more acutely 



