W. KEEPING OX EHE GENUS PELAXLXUIXUS. 925 



the month. The plates are yen' narrow and long, especially those at 

 the ambitus, and are bounded not by straight lines, but by curve! 

 or gently undulating edges ; the ambulacra! edge is crenate, with 

 three or four baylets. The plates do not overlap each other, but 

 both the upper and lower edge of each plate are slightly hollowed. 



The larger interambulacral plates bear three primary tubercles, 

 whose areas extend almost to the edges of the plate ; and also one or 

 two smaller primary tubercles near the median and zonal lines of 

 the (interambulacral) area. Xumerous minute perforated tubercles, 

 and still smaller granules, border the whole plate, form rings around 

 the larger tubercles, and crowd the spaces between them. The 

 plates near the oral pole bear three primary tubercles, and of these 

 the central one is largest. 



The ambulacral areas (fig. 2) are broad, and vary little in their 

 course round the test : so that, while they are nearly as broad as the 

 interambulacral areas at the oral extremity, at the equator they are 

 scarcely more than one third as broad. On the non-poriferous zone 

 are two ranges of primary tubercles, which are alternately larger 

 and smaller. The plates also are alternately large and small, the 

 greater being as broad, or nearly so, as the interambulacral plates, 

 whilst the small ones are only half that size. A consequence of this 

 difference in the plates is, that the usual zigzag line of junction of 

 the ambulacral plates is not produced here, but the angle- end of 

 each larger plate is truncated for the fiat face of a smaller ambu- 

 lacral plate in the other range opposite (fig. o). 



Every ambulacral plate carries one primary tubercle : and minute 

 tubercles and granules are present here just as they are upon the 

 interambulacral areas. 



The poriferous zones are straight and rather broad (about equal 

 to the non-poriferous zone), expanding slightly from the pole out- 

 wards. The pairs of pores (fig. G) are surrounded by a prominent 

 oval ring, which a slight depression renders double around the longer 

 sides. Xear the mouth the pores are arranged in double oblique 

 pairs ; elsewhere they are trigeminal. Ininute tubercles and granules 

 are scattered between the groups of pores. 



The actinal (buccal) area is \h inch iu diameter (7. e. three eighths 

 of the whole test), its circular outline being interrupted by ten broad 

 and deep peristomial notches. Its centre is occupied by the broken 

 ends of the jaws, and the rest of the area around these is covered 

 with large squamous plates which overlap each other regularly to- 

 wards the mouth (fig. 4). These bear tubercles in radiating series. 

 The exposed surface of each plate is very long transversely (nearlv 

 as long as the breadth of the whole ambulacral area close by) and 

 narrow, with rounded and irregular contours. Two or three pri- 

 mary tubercles are seen upon it which are smaller than those of the 

 corona ; and the minute tubercles and granules are also present, 

 though less abundantly than elsewhere upon the test. 



Only one third of each of these scale-like buccal plates (fig. 9) is 

 exposed, and this part is nearly as thick as a coronal plate ; but the 

 overlapped portion gradually thins away to an edge. 



