ACANTHASCUS CACTUS. 153 



the hump-like prominences are seldom seen, these being usually 

 either merely indicated or not at all present. The space where 

 they should occur may even be concave. It seems that the 

 prominences, as also the ridges arising from them, disappear as 

 the central node becomes larger with the growth of the entire 

 spicule ; possibly they become, so to say, covered up by the 

 siliceous deposit added to the node. The secondary principals, 

 which may measure 7* /^ across in the middle, are obsoletely 

 rough-surfaced. The terminals are similarly rough. Near their 

 outer end the roughness grows somewhat coarser and is here seen 

 to be caused by retroverted microtubercles (PL XIL, fig. 28). 

 In direct proximity to the terminal disc, which marginally runs 

 out into 7 or 8 small teeth, there exists a very short tract devoid 

 of the microtubercles. 



Not rarely malformed discoctasters are met with, in w^hich 

 some primary terminals have apparently failed to unite in the 

 proper way with any of the secondary principals but remain more 

 or less free, appearing like supernumerary appendages to an 

 ordinary discoctaster. As a case of such malformation is to be 

 considered the spicule figured by F. E. Schulze in the Challenger 

 Eeport, PI. LVIL, fig. 4. 



Of special interest are the isolated cases of true discohexasters 

 I have come across in certain specimens and which I consider 

 to represent the primitive form whence the discoctaster was 

 derived. In general size and in the appearance of the terminals 

 they are exactly compara1)le to the smaller discoctaster of the 

 periphery, but the essential difierence consists in the fact that 

 the principals are six in number, these being short, thick and 

 knob-like and arranged in the usual disposition. Four or five 

 terminals arise divergingly from each principal. The discohexaster, 



