152 THE VOYAGE OF H.M.S. CHALLENGEE. 



*Cystechmus ivyvilUi (PI. XXIX. figs. 5-8 ; Pis. XXIX.^ XXIX.'^ ; PI. XXXIX. 

 fig. 28 ; PI. XL. figs. 59, 60 ; PL XLI. figs. 23-27 ; PI. XLTI. figs. 13, 14 ; PI. XLV. 

 figs. 25-28). 



Cystechiims Wyvillii, A. Agassiz, 1879, Proc. Am. Acad., vol. xiv. p. 208. 



In a large specimen figured on Plate XXIX.* the outline of the test seen from above 

 is elliptical, broadly rounded anteriorly, and having the posterior extremity somewhat 

 more pointed than the anterior (PL XXIX.** fig. 1). Seen from the actinal side the 

 actinostome is transversely elliptical, eccentric, placed anteriorly ; the actinal surface is 

 quite flat. An indistinct, low, broad, actinal keel is formed by a slight swelling of the 

 posterior interambulacral space between the actinostome and the anal system. The 

 anal system is placed near the ambitus in a slightly bevelled plane, passing to the broad, 

 indistinct, rounded, abactinal anal hood (PL XXIX." fig. 3). The apical system is 

 slightly eccentric posteriorly, so that when seen in profile the anterior extremity slopes 

 out more gradually from the rounded apex, which is coincident with the apical system, 

 than the more suddenly sloping posterior extremity. 



The abactinal part of test, sloping uniformly for a short distance from the apex 

 (PL XXIX.* figs. 3, 4), forms a slight re-entering curve before it passes into the gibbous 

 ambitus, which, when seen in profile, is specially prominent in the anterior extremity of 

 the test owing to the eccentric position of the apex. Seen facing the odd posterior 

 interambulacrum the test has an uniformly arched and re-entering curve (PL XXIX." 

 fig. 4). This species has a comparatively stout test, and, to judge from the fragments of 

 some incomplete specimens from Station 296, it must have attained a diameter of at 

 least 130 mm., and a proportional height. The coronal plates are of very uniform size 

 (PL XXIX.* figs. 1-4), gradually becoming smaller towards the apex on the abactinal 

 surface, and towards the actinostome on the actinal surface, there is but little difference 

 in the size of the plates of the ambulacral system, and those of the interambulacral areas, 

 as in this genus the former are comparatively large. The odd posterior interambulacral 

 area is somewhat narrower than the other ; the lateral interambulacral areas are 

 slio-htly the broadest. Seen from the actinal side, the plates towards the ambitus 

 (PL XXIX.* fig. 2) become very much elongated, but become again quite hexagonal on 

 the actinal floor towards the actinostome. The whole test is covered with very small, 

 sharp, cylindrical, spines (PL XXIX. figs. 5-8), these and the mUiaries are sufiiciently 

 thick to hide completely the sutures of the plates, and the lines separating the 

 ambulacral and the interambulacral zones (PL XXIX. figs. 5-8). Minute pointed 

 pedicellarise are scattered irregularly over the whole test. 



Each coronal plate (PL XXIX." figs. 1-4, 9) carries but few primary tubercles, sup- 

 porting thin short delicate cylindrical spines, the intertubercular space being covered with 

 irregularly scattered miliaries (PL XXIX." fig. 9). Each coronal plate is also ornamented 

 by ridges radiating from the angles of the plate (PL XXIX. * fig. 9) towards the central 



