REPORT ON THE ECHINOIDEA. 195 



*Aceste. 

 Aceste, Wy. Thomson, 1877, The Voyage of the Challenger, Atlantic, vol. L p. 376. 



*Ac€Ste hellidifera (PL XXII. figs. 7-11 ; PI. XXXIII.'' figs. 1-7; PI. XXXIX. fig. 

 21 ; PI. XL. figs. 66-68 ; PI. XLI. figs. 13-16 ; PL XLIL figs. 26-28 ; PL XLIII. fig. 

 25 ; PL XLIV. figs. 45, 46). 



Aceste hellidifem, Wy. Thomson, 1877, The Voyage of the Challenger, Atlantic, vol. i. p. 37G, 

 figs. 95, 66. 



At first glance this appears one of the most remarkable of Sea-urchins. When seen from 

 the abactinal side (PI. XXXII. fig. 8) nearly the whole of the abaetinal surface is occupied 

 by the deeply-sunken, broad, odd anterior ambulacrum surrounded by a narrow elongated 

 pentagonal peripetalous fasciole, within which spring large flattened spathiform spines 

 curving over the few huge ambulacral suckers which fiU nearly the whole of the sunken 

 space of this area. The abactinal surface is quite flattened (PI. XXXIIL* figs. 4, 5), so 

 that when seen in profile (PL XXXII. figs. 9-11, figs. 9, 10, 11 are figured with the 

 actinal surface towards the upper side of the plate) the test is seen to arch regularly 

 towards both the anterior and posterior extremities (PL XXXIII. * fig.). The apical 

 system is placed just within the posterior edge of the peripetalous fasciole (PL XXXII. 

 fig. 8), from which the test slopes rapidly towards the actinal surface. The anterior 

 extremity is deeply indented (PL XXXII. figs. 7, 8, 9), the anterior part of the odd 

 ambulacrum running in a deep groove from the abactinal region to the actinostome 

 (PL XXXII. fig. 9 ; PL XXXIII.'' figs. 1, 5). 



The actinal plastron is large and prominent (PL XXXIII.* fig. 4) carrying long 

 spathiform spines (PL XXXII. figs. 7, 9-11) ; those which cover the test are shorter and 

 stouter (PL XXXII. fig. 8), while those surrounding the anal system (PL XXXII. 

 figs. 7, 10) are long, curved, and pointed. The spines on each side of the actinal groove 

 are closely crowded. Seen from the extremities (PL XXXII. figs. 9, 10) the test is 

 seen to arch regularly from the actinal keel in the centre of the plastron to the ambitus, 

 which in this genus is close to the flattened abactinal surface (PL XXXII. fig. 9 ; 

 PL XXXIIL* figs. 4, 5) ; the test is somewhat more rounded toward the posterior end 

 (PL XXXII. fig. 10 ; PL XXXIII.^ fig. 4). 



The enormous development of the suckers of the odd anterior ambulacrum is an 

 eminently embryonic feature, it exists in the youngest stages of all the Spatangoids of 

 which we know the development. In some species of Schizaster this character is also 

 found. In Aceste also, as in the Schizasteridae, we find the apical part of the anterior 

 lateral ambulacra made up of extremely narrow plates (PL XXXIIL* figs. 1,7), which 

 form the thin edge of the deep anterior groove, along the sides of which extend in Aceste 

 the simple anterior lateral ambulacra (PL XXXIII." figs. 1, 7), and in Schizaster proper 

 the more or less narrow anterior ambulacral petals placed within the peripetalous fasciole, 



