288 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



tubercles (of pedicellariae) between them, indeed often a regular 

 series. Within the marginal tubercle each plate usually carries one 

 or two much smaller tubercles at the lower edge, forming fairly regu- 

 lar longitudinal and, when two of them are present, horizontal series. 

 The rest of the interporif erous zone is naked ; sometimes, however, 

 there is a tubercle near the median corner of the plate, or there may 

 be two tubercles just within the marginal one, placed one at the 

 lower edge, the other higher up; in such cases the regular serial ar- 

 rangement of the inner tubercles is lost, and the naked character of 

 the interporif erous zone much reduced. The pores are mostly about 

 of equal size, but sometimes the inner pore is rather distinctly the 

 larger. They are rather distant, with the wall between them fairly 

 broad, low, and rounded; the ridge separating the adjoining pore pairs 

 is low and rounded. The pores, generally speaking, might almost be 

 termed semiconjugate (pi. 75, figs. 1-2). The pore zone is not 

 sunken. The ambulacral plates are low and rather numerous. There 

 is a considerable variation in the ambulacra. While the usual, and what 

 would appear to be the more typical and normal, condition is that the 

 space between the two series of marginal tubercles is fairly broad 

 and naked, this space sometimes is so narrow as scarcely to give room 

 for more than the one small tubercle within each marginal tubercle, 

 or it may even be wider than usual, so as to give room for a regular 

 series of up to five small tubercles at the lower edge of each plate. 



In the interambulacra there are 6, more rarely 7, coronal plates in 

 a series, the upper ones in each series more or less irregular, prolonged 

 upward at the adradial edge, often to such a degree that the upper- 

 most (rudimentary) plate is wholly excluded from the adradial edge. 

 The tubercles are in general small and inconspicuous, the boss low and 

 without a trace of crenulation. The areoles are only very little 

 sunken. At most the 2 or 3 proximal ones are confluent, but more 

 often even the lowermost ones are separated by a distinct series of 

 tubercles; the proximal ones are rather distinctly transverse-oval. 

 Above the ambitus the distance between the areoles usually increases 

 considerably, thus giving room for several miliary tubercles between 

 the neighboring scrobicular circles. The uppermost tubercle (and 

 correspondingly the uppermost primary spine) in each series is more or 

 less rudimentary, the areole widening irregularly on the adapical side, 

 becoming effluent and more or less indistinct. The scrobicular ring 

 of these upper areoles is likewise more or less rudimentary, extending 

 toward the adapical side and here often quite open, and evidently 

 obliterating. The scrobicular ring of the ambital tubercles is not 

 raised and is relatively inconspicuous, even though these tubercles 

 are about twice as large as the marginal ambulacral tubercles. Out- 

 side the scrobicular ring the plates are covered more or less closely 

 with very small miliary tubercles, leaving a fairly distinct naked 



