974 THE VOYAGE OP H.M.S. CHALLENGES. 



The genus Coronidium and the following, nearly allied Acanthodesmia, form 

 together the peculiar subfamily of Acanthodesmida not in the wider sense in which I 

 first founded this group (1862, Monogr. d. Kadiol., p. 265), but in the restricted sense, 

 which is exactly defined in my Prodromus (1881, p. 445). According to this definition, 

 the shell is composed of three different rings, perpendicular to one another ; only one of 

 these is complete, the simple horizontal basal ring ; the two others are incomplete and 

 vertical (the primary or sagittal and the secondary or frontal ring). Therefore there remain 

 constantly between the three rings five characteristic large openings or gates ; four of 

 these are lateral (between the halves of the two meridional rings), the fifth is basal, 

 enclosed by the horizontal basal ring. The longest known type of this subfamily is 

 Acanthodesmia vinculata, the five characteristic gates of which are clearly distinguished 

 by its discoverer, Johannes Miiller (" Das Gehause besteht nur aus den Leisten zwischen 

 fiinf grossen Liicken "). The Acanthodesmida may be derived from the Eucoronida by 

 reduction of the basilar rod of the sagittal ring. If in Eucoronis this basal rod be lost, 

 Coronidium arises. 



1. Coronidium dyostephanus, n. sp. (PL 82, fig. 7). 



Frontal ring elliptical, with few scattered thorns, twice as broad as high, and three times as 

 broad as the smooth rhombic basal ring. Sagittal ring semicircular, very stout, twice as thick as 

 the two other rings, with short lateral thorns (in the figure seen from the apical pole, which 

 exhibits a four-lobed dimple). Basal gate rhombic. 



Dimensions. Height of the frontal ring 0'08, breadth - 16. 



Habitat. Central Pacific, Station 263, depth 2650 fathoms. 



2. Coronidium diadema, n. sp. (PI. 82, fig. 8). 



Frontal ring kidney-shaped, one and a half times as broad as high, with a slight sagittal con- 

 striction. Sagittal ring ovate, smaller, about of the same size as the elliptical basal ring. Eods of 

 all three rings cylindrical, bearing numerous short and stout curved spines, partly simple, partly 

 branched. Four bunches of larger spines on the four corners of the basal ring, and one very large 

 bunch on the apical pole. Basal gate elliptical, one and a half times as broad as long. 



Dimensions. Height of the frontal ring 013, breadth 0'18. 



Halitat. Indian Ocean (Madagascar), Eabbe, surface. 



3. Coronidium cervicorne, n. sp. (PI. 82, fig. 1). 



Frontal ring kidney-shaped, twice as broad as high, one and a half times as broad as the 

 elliptical basal ring, both with a slight sagittal constriction. Sagittal ring ovate. All three rings 



