REPORT ON THE RADIOLARTA. 749 



Genus 325. Acanthoma,^ Haeckel, 1881, Prodromus, p. 465. 



Definition. — As tr olo ncliida with simj^le, four-edged, prismatic or pyramidal 

 radial spines, without apophyses ; their transverse section is square. 



The genus Acanthoma comprises all those Astrolonchida (formerly united with 

 Acanthometron) in which the simple spines either in their whole length or in the 

 greatest part of it are four-edged, with square transverse section. They are sometimes 

 more prismatic (with equal breadth), at other times more pyramidal (with decreasing 

 breadth towards the distal apex). If Acanthometron be the common simple ancestral form 

 of the Acanthonida, then the two-edged Zygacantha and the four-edged Acanthoma may 

 be regarded as two divergent main lines arising from it ; the former leading to the Phracta- 

 canthida and Diporaspida, the latter leading to the Stauraeanthida and Tessaraspida. 



Subgenus 1. Acanthonarium, Haeckel. 



Definition. — Spines at the central base without leaf-cross and without hollow 

 pyramidal compartments, united by the triangular faces of their pyramidal bases, resting 

 one upon another. 



1. Acanthojiia tetracojxi, Kaeckel {F\. 129, figs. 9-11). 



Aeanthoinetra tetracopa, J. Miiller, 1858, ALhandl. d. k. Akad. d. Wiss. Ijedin, p. 17, Taf. vii. 



figs. 3 to 5, Taf. xi. fig. 5. 

 Aeanthoinetra iefracopa, Haeckel, 1862, Monogr. d. Eadiol, p. 379, Taf. xviii. fig. 5. 



Spines quadrangular prismatic, with prominent lamellar edges, of equal breadth in their 

 whole length. Base pyramidal, without leaf-cross. Apex truncated, or with four short teetli 

 (sometimes only two teeth). Central capsule opaque, brown or yellow. 



Dimensions. — Length of the spines 0T5 to 0'2, breadth O'Ol to 0'012. 



Habitat. — Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, many Stations, surface. 



2. Acanthoma prismatica, n. sp. 



Spines quadrangular prismatic, without prominent edges, with four pilane lateral faces, of 

 equal breadth throughout their whole length. Base pyramidal, without leaf-cross. Apex truncate, 

 with square apical face. 



Dimensions. — Length of the spines 01 to 0'16, breadth 0'005 to O'OOS. 



Habitat. — Antarctic Ocean, Station 154, surface. 



' Acanlhonia = &))my article ; clHavlx.. civicc. 



