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



Phseospheeria (Phceosphceria articulata), on the other hand, the lattice-sphere is 

 segmented in quite a peculiar manner, and composed of hollow cylindrical tangential 

 tubes, which are separated by astral septa at the nodal points of the network ; this remark- 

 able structure characterises the two families, Aulosphserida (Pis. 109-111) and Canno- 

 sphaerida (PL 112); the segmented lattice-sphere of the former is simple and hollow ; 

 while that of the latter is connected by centripetal radial tubes with a simple concentric 

 inner shell, which is sometimes solid, sometimes latticed, and provided with a main- 

 opening corresponding to the astropyle of the enclosed central capsule. Since in the 

 Aulosphserida also, hollow centripetal radial tubes project from the segmented lattice- 

 sphere, it is possible that they have been derived from the Cannosphserida by the loss of 

 the primitive internal shell. A special peculiarity of many Phseosphseria (Oroscena, 

 Sagoscena, Auloscena, &c.) consists in the fact that the whole surface of the lattice- 

 sphere is regularly covered with pyramidal or tent-shaped prominences (PI. 106, fig. 4 ; 

 PL 108, fig. 1; PL 110, fig. 1). A simple lattice-sphere quite similar to that of most 

 Monosphserida also constitutes the skeleton of the Castanellida (PL 113), but since it 

 possesses a special main-opening, it must be referred promorphologically to the Cyrtoid 

 shells of the Pheeogromia. 



120. The Prunoid Skeleton or Lattice-Ellipsoid. The "lattice-ellipsoids " or Prunoid 

 skeletons have arisen from the lattice-spheres or Sphaeroid skeletons by more energetic 

 growth and elongation of one axis ; this is the main axis of the body and is probably 

 always vertical; its two poles are commonly equal. The Prunoid skeleton is either 'a 

 true ellipsoid in the geometrical sense or an " endellipsoidal polyhedron" (i.e., a poly- 

 hedron, all the angles of which lie in an ellipsoidal surface). By further elongation of 

 the main axis, the ellipsoidal form passes over into the cylindrical, the polar surfaces of 

 the cylinder being usually rounded, rarely truncated. The rich order Prunoidea 

 (pp. 284-402) contains numerous modifications of this form of shell which arise on the 

 one hand by the formation of transverse constrictions, on the other by the apposition of 

 concentric secondary shells. In respect of the latter, simple and compound Prunoid 

 shells can be distinguished as in the case of the Sphseroid shells. In the compound 

 Prunoid shells either all the concentric lattice-shells may be ellipsoidal or the inner may 

 be spherical. More important differences are found in the transverse annular constric- 

 tions, which give the Prunoid skeleton a segmented appearance ; in this respect, three 

 principal forms may be distinguished (p. 288) : (A) Monoprunida, with unsegmented 

 shell, having no transverse constriction (Pis. 15-17); (B) Dijoprunida, having a shell 

 with two segments and one (equatorial) transverse constriction (PL 39) ; (C) Poly- 

 prunida, with three or more parallel transverse constrictions, by means of which the 

 shell is divided into four or more segments (PL 40). In the same manner as the 

 Prunoidea have arisen from the Sphseroidea among the SPUMELLARIA by greater 



