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



Genus 212. Archidiscus,^ n. gen. 



Definition. — P o r o d i s c i d a with a simple central chamber, surrounded by a single 

 concentric ring, which is divided by radial beams into two to six or more radial chambers, 

 without radial spines on the margin. 



The genus Archidiscus begins the long and polymorphous series of the Cyclo- 

 discaria or of those Discoid ea which do not possess the peculiar "phacoid 

 shell" characteristic of the three preceding families, united as " Ph ac odiscaria." 

 As already mentioned above, both these groups are probably of independent origin, derived 

 from the Sphseroidea in different ways (compare pp. 402, 405, &c.). Among all 

 Cyclodiscaria Archidiscus is the most simple, and probably the common ancestral 

 form, from which the other genera may lie derived. 



Subgenus 1. Dioniscus, Haeckel. 

 Definition. — Ring with two chambers, separated by two radial beams. 



1. Archidiscus dioniscus, n. sp. 



Ring circular, connected with the central chamber by two radial beams, opposite in one axis, 

 therefore two equal semicircular ring chambers. (This primitive form has an interesting reference 

 to Saturnalis, PI. 13, fig. 16, and differs from it only in the lattice- work covering both faces of the 

 lenticular disk, the margin of which forms the ring.) 



Dimensions. — Diameter of the ring O'Oo, of the central chamber 0'016. 



Hahitat. — Central Pacific, Station 271, depth 2425 fathoms. 



2. Archidiscus dithalamus, n. sp. 



Ring roundish, connected with the central chamber by two radial beams, not opposite in one 

 axis; both semicircular ring chambers more or less unequal, one of them larger than the other, and 

 sometimes much more prominent. (If this prominence increase, we can regard it as the beginning 

 of spiral convokitions, Discospira.) 



Dimensions. — Diameter of the ring 0'06, of the central chamber 0'014. 



Hahitat. — Central Pacific, Station 266, depth 2750 fathoms. 



Subgenus 2. Trioniscus, Haeckel. 

 Definition. — Ring with three chambers, separated by three radial beams. 



' Archidiscus ='Pnmor&iid disk ; d^x'^iaK^g. 



