196 THE VOYAGE OF H.M.S. CHALLENGER. 



while the short posterior lateral ambulacral petals of Schizaster are replaced in Aceste by 

 the three or four ambulacral plates perforated with simple pores, which are placed within 

 the so-called peripetalous fasciole ; this fascicle affecting in no way the structure of the 

 ambulacral jjlates placed within its limits. 



The general outline of the test as seen both in profile and from above is strikingly 

 similar to that of the Schizasteridse. In fact, this genus is of the greatest interest, showing 

 as it does striking afiinities on the one side to the Schizasteridse and other Spatangina, such 

 as Brissopsis, and on the other to the Pourtalesias, not only in the structure of its ambulacral 

 system, but also from the position and shape of the actinostome, and the more or less 

 cylindrical test modified in its outline from its Schizasterid afiinities. This is well seen 

 in the end views of the test (PI. XXXIIL* figs. 4, 5), the anterior part of which is more 

 angular, as in the Schizasteridse, while the posterior is more cylindrical, as in the Pourta- 

 lesifB and Brissina. 



Seen from above the tuberculation of the test is quite uniform (PJ. XXXIIL* fig. l),with 

 the exception of the ambulacral zones within the fasciole, which consist of smaller tuber- 

 cles. Seen from the actinal side the actinal plastron (PL XXXIII.'* fig. 2) is covered by 

 large primaries with a flat areolar space, with large primary tubercles extending over the 

 anterior part of the test mainly in the interambulacral areas adjoining the actinostome 

 (PI. XXXIII." figs. 3, 5). The tuberculation of the posterior part of the test is smaller, 

 and the tubercles are more closely packed (PI. XXXIII.'' figs. 2, 3, 4). 



The anal system (PL XXXIII." figs. 2, 4) is placed on the steep sloping posterior 

 extremity of the test about half-way from the level of the actinal plastron to the edge of 

 the fasciole. It is circular, slightly pointed vertically ; the anal opening is near the lower 

 edge of the anal system. The anal system is covered with small elongated plates 

 arranged in more or less irregular concentric rows round the anal opening. Seen from 

 the actinal side the anal system appears on that side of the test (PL XXXIII.''' fig. 2). 

 The largest specimen collected measured 27 mm. in length, the smallest 14 mm. 



In Aceste there are in the broad odd amljulacral field within the poriferous zone of 

 the abactinal system a few minute spines (PL XL. fig. 66), which are interesting as pre- 

 senting a modification of the tip of the spine not hitherto observed. It is a change from 

 the usual flattened, paddle-shaped, or club-shaped tip characteristic of the spines of so 

 many Spatangoids to form an umbrella-like hood. This hood is formed by a ring of large 

 serrations projecting far beyond the general outline, and gradually passing again both 

 above and below into the regular reticulation of the spine. The only similar structure 

 known in spines is that of the Ophiuran genera Oi^hiohdus and 02Jh{othoUa, collected by 

 the " Challenger" and by the " Blake," where, owing to the small number of cells composing 

 the spine, this umbrella-like structure is more apparent than it is in Aceste. This pecuhar 

 spine of Aceste is also interesting as showing a possible transition from normal to more 

 specialised spines, which may* in part perform the functions of pedicellariae. 



