38 PROF. p. M. DUISTCAK A-RD MK. W. P. SLADElSr 0"N 



in well-preserved specimens. On a fully developed plate there is 

 tlie more or less continuous median ridge already noticed, with 

 some separate granules, conical in shape, near the aboral point of 

 the plate and breaking the continuity of the ridge. On either 

 side of this median ridge are two or more raised ridges, separated 

 by sunken or shallow grooves, placed obliquely and directed out- 

 wards and ujjwards to the suture between the radial and basal 

 plates. The ridges and furrows are continuous with those of the 

 Basals, the suture forming a marked thin line of separation. These 

 ridges may be continuous over the radial plate, or they may.be 

 interfered with by the presence of granules along their line. A 

 ridge is usually on the flank of the radial plate close to the side 

 edge which is free and adoral to that edge within the Basals. 

 This ridge, seen on both sides of the plates, need not be conti- 

 nuous with any structure of the Basals. 



IV. The Apical Sutures. 



These are beneath very distinct grooves between the Basals and 

 Eadials and between the Basals themselves. The grooving inter- 

 feres with the continuity of the ridges and furrows but does not 

 alter their direction. 



In some specimens there is a decided fibrous appearance of the 

 surface of the test in the vicinity of the sutures, and a more or less 

 regular dove-tailing of the angular points of the fibres of one 

 plate occurs with those of the other across the line of suture. 

 This structure also occurs between the sutures of the interradial 

 plates just at the surface of the grooves. 



V. The Ohliq^uity of the Interradial Flates. 



This is very marked, and the transverse sutures of the inter- 

 radia do not make right angles with an imaginary line passing 

 vertically down the median area (PL I. fig. 2). The obli- 

 quity of the pairs of pores has been noticed, and it appears that 

 the direction refers to the obliquity of the interradial plates, which 

 pass, not straight across the space, but in a direction upwards 

 and inwards. About midway between the apical system and the 

 ambitus there is a decided obliquity of the outer third of each 

 interradial plate, and the vertical zone of granules and small pri- 

 maries close to the ambulacra is placed on this oblique part. 



