742 THE VOYAGE OF H.M.S. CHALLENGES. 



Subfamily 1. ZYGACANTHIDA, Haeckel. 



Definition. A strolonchida with twenty simple radial spines, without apophyses 

 or lateral transverse processes. 



Genus 323. Acanthometron, 1 J. Muller, 1855, Monatsber. d. k. preuss. Akad. 



d. Wiss. Berlin, p. 229. 



* Definition. A strolonchida with simple cylindrical or needle-shape'd radial 

 spines, without edges and without apophyses ; their transverse section is circular. 



The genus Acanthometron, with the restricted definition here given, is the most simple 

 form of all Acanthonida, and may be regarded as the common ancestral form not 

 only of this suborder but also of all A c a n t h o p h r a c t a, in general of all Icosacantha, 

 or all ACANTHARIA in which twenty radial spines are regularly disposed after the Mullerian 

 law (p. 717). In the wider sense, given originally to Acanth ometr a by Johannes 

 Muller, its discoverer, this genus comprised all ACANTHARIA constituting here our order 

 "Acanthometra" (Radiolaria without lattice-shell, with radial spines united in the 

 centre). In my Monograph (1862, p. 375) I restricted this genus to those " Acantho- 

 metrida" in which twenty simple spines of equal size (and without apophyses) are supported 

 one upon another in the centre, and I separated as Astrolithium those forms in which 

 they are grown together in the centre. But this difference now appears not so important, 

 and I restrict here the genus Acanthometron (not Acanthometra) to those most 

 simple forms in which the simple radial spines are cylindrical or conical, without edges. 



Subgenus 1. Acanthometrella, Haeckel. 



Definition. Spines at the central base without leaf-cross, united by the opposed 

 triangular faces of their pyramidal bases, resting one upon another. 



1. Acathometron elasticum, Haeckel. 



Acanthometra elastica, Haeckel, 1862, Monogr. d. Eadiol., p. 376, Taf. xv. fig. 1, Taf. xviii. fig. 1. 

 Acanthometra elastica, B. Hertwig, 1879, Organismus d. Eadiol., Taf. i. figs. 2, 2, 2&. 



Spines cylindrical, very thin and long, needle-shaped, at the central base four-sided pyramidal, 

 without leaf-cross. Distal apex conical. The spines are very elastic, of nearly equal thickness in 

 their whole length. Central capsule quite pellucid, colourless, with a variable number of yellow 

 pigment-bodies (xanthellffi ?). 



Dimensions. Length of the spines 0'3 to 0'6, breadth O'OOl to 0'002. 



Habitat. Cosmopolitan, very common in all warmer _seas; Mediterranean, Atlantic, Indian, 

 Pacific, surface. 



1 Acanthometron = Spine proportion ; uxxvQa, /xir^oa. 



