from the Louer Greenmnd. 533 



the main series. Collerette long, often one-third the length of the 

 radiole, smooth, and separated from tlie stem hy a prominent, slightly 

 ohlique 'bourrelet'. Annulus prominent, base strongly developed. 

 Acetabulum deep, with a crenelated margin and large central 

 perforation. 



Description : General Characters. — Except in. a few cases all the 

 portions of the test have been separated (often being waterworn), so 

 that an accurate idea of the form of the urchin cannot be obtained. 

 Judging from the very slight curvature of four associated plates of one 

 column, and from the large dimensions of plates from the adapical 

 part of the test, it may be inferred that the whole body was large and 

 tall, perhaps somewhat pyriform. The diagnosis and description of 

 the plates are generalized from over four liundred specimens, and of 

 the radioles from nearlj^ six hundred. The examples chosen for 

 figuring show average types, among which may be included Wright's 

 original figures. 



The Apical System. — The genital plates (PI. XXV, Fig. 10) are 

 almost square in outline, the specimen figured measuring 4 mm. 

 along the periproctal and radial margins. The thickness increases 

 from the periproctal edge adorally, and at the coronal margin the 

 plates are abruptly truncated. The genital pores are large and 

 circular, placed at a distance of about one-quarter the radial diameter 

 of the plates from the coronal edge. They pass obliquely through 

 the plates, emerging in a slight concavity near the centre of their 

 inner surfaces. The external ornament of the plates consists of 

 granules on a shagreened surface. One row of granules, larger than 

 the others, forms a rim between the genital pore and the coronal 

 margin, while the rest are engaged in close-set lines, which tend to 

 radiate from a point to one side of the pore. The plate-surfaces 

 become smooth towards the edges, especialh' that which borders on 

 the periproct. 



The ocular plates (PI. XXV, Fig. 11) are similar to the genital ones, 

 except in shape. They are elongated transversely, and are strongly 

 concave along the periproctal margin, being convexly rounded at the 

 coronal edge. They too show a loss of ornament near the periproct. 



The Corona: Interamhulacral Plates. — Average-sized plates from 

 the adapical surface of the test measure about 13"5 mm. longitudinally 

 and about 10 mm. along the adoral transverse margin. The plates 

 are constantly taller than broad, sharply incurved at the adapical end, 

 scarcely curved in section in the other parts. The adapical transverse 

 margin is a little inclined towards the interradial suture, and slightly 

 concave. The adapical oblique margin is concave to a less degree, but 

 the other three edges are sensibly convex. The adradial margin is 

 strongly curved, concave adapically, but convex (concentrically with 

 the scrobicular ring) in the middle and adoral parts. It is furrowed 

 on the adradial surface by about twenty-four grooves. The scrobicule 

 is wide, circular, slightly excavated, and situated towards the adoral 

 region of the plate, near to the adradial margin. There is a definite 

 basal terrace at about two-fifths the radial distance from the ring. 

 The boss is central, rising like a dome to a platform of a diameter 

 one-third that of the scrobicule. The platform is very slightly 



