1146 THE VOYAGE OF H.M.S. CHALLENGER. 



Subfamily 2. Archiperida, Haeckel, 1881, p. 429. 



Definition. — Tr ip o c al pid a with the basal mouth of the shell fenestrated {vel 

 Monocyrtida triradiata clausa). 



Genus 504. Euscenixim^ n. gen. 



Definition. — A r c h i p e r i d a (vel Monocyrtida triradiata clausa) with a free 

 simple internal columella, prolonged into an apical horn. Three basal feet free, without 

 lateral wings. 



The genus Euscenium and the three following nearly allied genera form together 

 the remarkable small group of Euscenida, probably one of the oldest and most primitive 

 of all C y r t o i d e a, and perhaps the common root of this suborder, arising directly 

 from the P 1 e c t o i d e a. The simple skeleton is composed of four radial beams 

 united in one point (the cortinar centrum). Three beams are directed downwards, and 

 form three divergent basal feet ; the fourth beam is directed upwards and represents an 

 apical horn ; its basal part is enclosed as " columella" in the cavity of the simple shell, 

 the lattice-work of which connects the four spines (compare above, Plectaniscus, p. 924). 

 In the genus Euscenium, the simplest of all, the columella is simple, and the three feet 

 also simple, without wings. 



Subgenus 1. Euscenarium, Haeckel. 

 Definition. — Three feet simple, not branched. 



1. Euscenium plectaniscus (PI. 98, fig. 1). 



Shell three-sided pyramidal, thorny, about as long as broad, with three dentate edges. Frame- 

 work very loose, with irregular polygonal meshes ; in the middle of each of the three triangular 

 sides a horizontal band of denser network. Basal plate with three very large simple collar-holes. 

 The apical horn and the three feet are three-sided pyramidal, about half as long as the columella, 

 with three prominent irregularly dentate edges. The horn is straight, whilst the three feet are 

 hook-shaped, its distal end being curved downwards and inwards. 



Dimensions. — Shell Oil long, 013 broad. 



Habitat. — Central Pacific, Station 274, depth 2750 fathoms. 



2. Euscenium archicyrtis, n. sp. 



Shell three-sided pyramidal, smooth, nearly as long as broad, with three straight edges 

 connected by irregular loose framework with polygonal meshes. Basal plate with three very large 



' Euscenium = 'Nice small tent ; sJ uxni/zls. 



