AILEOSOMA THETIDIS. 177 



The other actinal coronal plates each carry one or two primary tubercles ; 

 these vary greatly in their position on the plate, but as a rule every other 

 plate has one such tubercle near its outer end and a second near the inner 

 end ; the alternating plates usually have a single large tubercle at the 

 middle, but occasionally there are two tubercles present. Sometimes a plate 

 occurs with no primary tubercles. It will be seen, therefore (PI. 69), that 

 there is a tendency to form three well-spaced longitudinal series in each half 

 of the interambulacrum. On the abactinal surface most of the coronal 

 plates carry no primary tubercles, but six or eight plates in each column 

 are made conspicuous (PI. 68) by the single large tubercle which each bears. 

 The inner, imbricating ends of the coronal plates are abactinally quite bare 

 and perfectly smooth (PL 68), but there is no uncalcified membrane between 

 the pjates, except for a very small area at the lower margin of about a 

 dozen plates, beginning with the fifth or sixth from the genital plate. 

 Actinally the coronal plates are well covered, clear to the median line, with 

 secondaries and miliaries, but abactinally the margins of each plate are, on 

 all sides, more or less bare. 



Each half-column of an ambulacrum actinally (PI. 69) is made up of very 

 wide, rather large primary plates, each accompanied by two small secondary 

 plate-elements. The latter are very little larger than the peripodium which 

 each bears. Although the tubercles are arranged in two series, on each 

 side, their distribution is quite irregular. It is rather more common to find 

 two tubercles on a single plate, one at each end, with the adjoining plates 

 above and below without tubercles, than to find them alternating, as might 

 be expected, plates with a tubercle at the inner end succeeding and being 

 followed by plates with an outer tubercle. Abactinally (PL 68) only five or 

 six plates in each column bear large tubercles, and these are irregularly 

 scattered. The remaining surface of the ambulacral plates is fully covered 

 actinally with secondaries and miliaries, but abactinally the margins of each 

 plate, especially the outer ends, are quite smooth and bare. The three series 

 of pore-pairs run rather close together the full length of the ambulacrum ; 

 even just above the ambitus (PL 70, fig. 4) the outer series (in the primary 

 plates) is not very widely separated from those in the secondary plates. 



The abactinal system (PL 70, fig. 1) is small and well defined. The 

 genital and ocular plates are not in contact with one another. The genitals 

 are long, triangular, separating the two upper pairs of plates of each inter- 

 ambulacrum ; the pores are large, occupying the greater part of the distal 



