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



latter, proposed in the Prodromus, were not quite suitable, I replace them here by the 

 following more convenient names : 



1. Family Stephanida ( = Monostephida, 1881, p. 447). Skeleton entirely formed 



by one simple ring (the primary vertical sagittal ring). The only " gate " 

 is the simple aperture of the ring. 



2. Family Semantida ( = Dyostephida, 1881, p. 446). Skeleton composed of 



two rings, perpendicular to one another ; the primary vertical sagittal 

 ring bears at the base a horizontal basal ring ; between the two rings 

 two or more " basal gates remain." 



3. Family Coronida ( = Triostephida, 1881, p. 445). Skeleton composed of two 



crossed vertical or meridional rings, perpendicular to one another the 

 primary sagittal ring and the secondary frontal ring. Commonly the two 

 vertical rings are united at the base by a horizontal basal ring, and 

 between them remain two or more " basal gates." 



4. Family Tympanida ( = Parastephida, 1881, p. 446). Skeleton composed of 



two parallel horizontal rings, an upper mitral ring, and a lower basal ring, 

 both connected by vertical or divergent columellae, which are parts of 

 vertical rings (primary sagittal, and secondary frontal ring). 



The peculiar structure of the central capsule of the Stephoidea, and their 

 character as true MONOPYLEA, were first recognised by Eichard Hertwig, who in 

 1879, in his Organismus der Eadiolarien, gave an excellent detailed description of it 

 (loc. cit., pp. 6872, Taf. vii. figs. 4, 5). He also pointed out the near affinity of these 

 " Acanthodesmida " with the Spyroidea or Zygocyrtida, uniting the latter with the 

 former family. 



A fuller explanation of this affinity, and of the great morphological and phylo- 

 genetic importance of the " Acanthodesmida," as ancestral forms of the Spyroidea 

 and Cyrtoidea, was given in 1882 by Butschli (Zeitschr. f. wiss. Zool., vol. xxxvi. 

 pp. 495501). He described in detail some important fossil forms of Acanthodesmida 

 as different species of " Stephanolithis," a name which Ehrenberg had employed for 

 various ring-like fragments of Eadiolarian shells, sponges, and other fossil bodies. 

 The four fossil species which Butschli described represent four different genera of 

 Stephoidea, viz., Semantis (spinescens), Semantrum (mulleri], Semantidium 

 (haeckelii), and Tristephanium (hertivigii). On the phylogenetic conclusions, derived 

 from these accurate observations, compare above, p. 893, &c. The topographical signifi- 

 cation of the parts, employed by Butschli, is contrary to mine ; he calls my dorsal 

 side the " anterior," and my ventral side the " posterior." 



The geometrical fundamental form of the body in nearly all Stephoidea (with 

 few exceptions) is distinctly bilateral or " dipleuric," so that we can easily distinguish the 



