1540 THE VOYAGE OF H.M.S. CHALLENGER. 



astropyle of the latter, placed on the oral pole of the main axis, corresponds to the 

 mouth of the shell in all P h se o g r o m i a. In the Phseosphseria, where no 

 peculiar shell-mouth is developed, the general fundamental form of the shell is usually 

 homaxonial or spherical, often an endospherical polyhedron, rarely ellipsoidal or spindle- 

 shaped (with prolonged main axis), or lenticular (with shortened main axis). The 

 bivalved Phseoconchia have usually either an amphithect shell (with the same 

 fundamental form as the Ctenophora), or a dipleuric, bilaterally symmetrical shell (with 

 a dorsal and a ventral valve, a right and a left parapyle). A small number of 

 PH^EODARIA (mainly Circoporida) are remarkable on account of the regular polyhedral 

 form of their shell, the geometrical axes of which resemble crystalline axes and are 

 denned by regular radial tubes; as the octahedral Circoporus (PL 117, fig. 6), 

 the dodecahedral Circorrhegma (fig. 2), and the icosahedral Circogonia (fig. 1). 



The siliceous or silicate bars, which compose the skeleton of the PH^EODARIA, are in 



the majority hollow tubes, filled up by jelly ; in some other families, however, they 



are solid rods, as in the NASSELLARIA and SPUMELLARIA. Such usual lattice-work, 



composed of solid rods, occurs only in the families Sagosphserida (PI. 108), Castanel- 



lida (PI. 113), and Concharida (Pis. 123-125). A quite peculiar structure, a 



diatomaceous tracery of extremely fine and regular hexagonal frames, distinguishes the 



Challengerida (PI. 99). The hollow cylindrical tubes, which are found in the other 



families, appear in three different forms, simple, articulate, and provided with an axial 



thread. Simple hollow tubes, which are neither articulate, nor provided with an axial 



thread, occur in the Cannorrhaphida (PL 101) Aulacanthida (PL 102-105), 



Coelodendrida (PL 121, 122), and Ccelographida (PL 126-128). In all these 



families the hollow cylindrical tubes have a very thin wall and contain a wide cavity, 



filled only by jelly. The Orosphserida (PL 106, 107), differ in the reduction of the 



cavity, which becomes very narrow (often rudimentary or lost), whilst the walls of the 



tubes become extremely thickened and stratified, numerous concentric 'layers of silica 



being disposed one over the other. The hollow cylindrical tubes contain an axial 



filament, or a thin thread of silica, placed in its axis, in the families Aulosphserida 



(PL 109-111), Cannosphserida (PL 112), Circoporida (PL 114-117), and Tusca- 



rorida (PL 100). Usually the axial filament is connected with the thin wall of 



the tube by numerous horizontal branches. A quite peculiar structure distinguishes 



the Medusettida (PL 1 18-120) ; their hollow tubes, extremely prolonged, are articulate 



owing to the presence of numerous, regular, equidistant transverse septa ; these 



are pierced by a short tubule, similar to the siphon of the shells of Nautilus ; this 



remarkable alveolar structure also occurs in the peripheral part of their shell-wall 



(and sometimes in the whole shell), numerous small polyhedral chambers or alveoles 



which communicate by small openings, being developed ; they become easily filled 



with air in the dry shell (PL 120, figs. 11-16). 



