64 Prof. P. M. Duncan on the Salenidge. 



primaries. Close to the apical system, in the interambulacra, 

 are usually extensions of the median system of small tubercles 

 and granules ; and they are most numerous above the pri- 

 maries which are remotest from the disk's edge. The apical 

 disk, irregularly pentagonal in shape, is larger than the peri- 

 stome, but is small in relation to the test. It by no means 

 covers the whole abactinal surface of the test ; and its sur- 

 face is nearly flat, the only elevation being in the region of the 

 subanal (preanal) plate, which slopes up to the anal orifice. 

 The ratio of the diameter of the apical system to that of the 

 test diminishes with age ; and the youngest specimen has the 

 disk more convex than those of the others. The oblique anal 

 opening, as a whole, is more elevated than the rest of the disk ; 

 and its raised edges include an irregular pentagonal area 

 whose long diameter is parallel with " the free edge of the 

 nearest ocular plate (the right posterior) . The sharp, rounded, 

 thin, irregular anal edge is most distinct in the young speci- 

 men, and is composed in every one by the joined everted 

 edges of the plates composing and surrounding the orifice. 

 The plates infringed upon by the anus and composing its 

 edge are the right posterior generative and the posterior gene- 

 rative plates and the sub- or preanal plate ; for the madreporic 

 plate or the right anterior, the left anterior, and left posterior 

 or lateral generative plates do not reach the orifice. 



The three anterior generative plates (that is to say, the ma- 

 dreporic, the left anterior, and the left posterior or lateral) are 

 the largest in the young specimen ; but with age the right 

 posterior becomes equal to the madreporic ; and the other two, 

 the right posterior and the posterior, are different in shape from 

 the others. There is but slight difference in the relative sizes 

 of the larger plates. The madreporic plate is seven-sided, is 

 roundly pointed externally, where it infringes slightly on the 

 median interambulacral space ; the plate is longer than 

 broad, the greatest breadth being internally. The free edge 

 is slightly waved. The sutural margins are distinct and 

 linear ; and the pits are round or lozenge-shaped in outline, 

 deep, and become more numerous with age. There is always 

 one at each sutural angle, and an extra one in the midst of the 

 antero-posterior suture, from the rear of the anterior ocular 

 plate to the preanal ; but in most of the specimens there is 

 also one in the suture which divides the plate from the right 

 posterior. The madreporic body has, as usual, been lost ; but 

 the scar is large and elongate. The left anterior plate re- 

 sembles the madreporic, has a well-marked central generative 

 pore ; and there is a pit in the midst of its suture with the pre- 

 anal. The left posterior plate, slightly smaller, has a well- 



