87 



so much (especially by their large anal plate) from those from Muscat that I could 

 not doubt that they belonged to a distinct species. The rich material of Pleurechinus 

 from the „Siboga'-Expedition, however, yielded so many forms intermediate between 

 the specimens from New Britain and Muscat that I must regard these as the two 

 extreme forms of one very variable and widely distributed species. In the „Siboga"- 

 Ech. p. 202 this species is mentioned under the name of Pleiirech. reticulatiis, as I 

 could not at that time say with certainty whether it was identical with PI. scillœ 

 or not, having not yet seen the type specimen. 



The test is low (6 mm. high to a diameter of 9 mm.), beautifully rounded 

 above, almost flat below. The pits (in both areas) are large, reaching to the base 

 of the primary tubercles, leaving only a small part of the horizontal suture not 

 deepened (PI. II. Figs. 4, 13); they are generally narrowed in the middle, rounded 

 at both ends, the end looking towards the primary tubercle being the wider; the 

 ambulacral pits are more regular in outline. There is a distinct covering mem- 

 brane round the edge of the pits. The surface of the plates looks very different in 

 the extreme forms of the species; but intermediate conditions are found. In those 

 from New Britain the primary tubercles (in both areas) are little conspicuous, some 

 few of those at the ambitus being a little larger than the others; their base is very 

 distinctly stellate, undermined, as it were, by a circle of deep, round pits. Along 

 the middle of the plate there is a depression in continuation with the pit in the 

 opposite suture. The plates thus get a very reticulated surface (PI. II. Fig. 13). The 

 secondary tubercles, which are of about the same size as the primary ones, are 

 arranged in a more or less distinct zigzag manner, forming oblique transverse series 

 between the primary tubercles of the two series. Outside the primary interambu- 

 lacral tubercles there are a few secondar}' tubercles. No vertical series are formed 

 by the secondary tubercles. — In the specimens from Muscat the primary tubercles 

 are more distinct, with only faint traces of a stellate condition of their base. There 

 is no distinct median depression on the plates, and the secondary tubercles are not 

 arranged in oblique series. — The pores are small, disposed in a straight line, close 

 to the edge. The poriferous zone is very narrow, only c. ' V as broad as the inter- 

 poriferous zone. The plates of both areas are equally high, their number being the 

 same (9 in a specimen of 8 mm. diameter). No pits are seen along the edge of the 

 ambulacral areas. 



The apical system (PI. II. Figs. 10, 12) in the specimens from New Britain 

 shows the peculiar feature of the periproct being completely covered by one_ large 

 anal plate, no small plates being found besides it. In other specimens some few 

 small plates are found on one side, and in the specimens from Muscat several 

 small plates are found, the anal plate being, however, very large. The pits at the 

 inner angle of the ocular plates is large, generally divided in two or three almost or 

 quite separate pits. The ocular pore is directed outwardly, generally not seen from 

 above. The genital plates have some more or less distinct depressions, the outline 



