REPORT ON THE RADIOLARIA. Ixxxi 



development of the vertical main axis, the ellipsoidal Belonaspida have arisen from the 

 spherical Dorataspida among the Acanthaeia (p. 859; PI. 136, figs. 6-9; PL 139, 

 figs. 8, 9). The main axis of the ellipsoid in this case is always occupied by the opposite 

 equatorial spines of the hydrotomical axis (pp. 719, 860). In the legion Ph^eodaria a 

 similar pi'olongation of the main axis rarely occurs ; it is found, however, in Aulatractus 

 (PI. Ill, iigs. 6, 7), the lattice-shell of this Aulosphserid being sometimes truly fusiform , 

 sometimes rather ellipsoidal or even double-conical. 



121. The Discoid Skeletons or Lattice-Discs. — The "lattice-discs" or Discoid 

 skeletons are characteristic of the Spumellaeian group D i s c o i d e a, and have arisen 

 from the lattice-sjjheres of the S p h se r o i d e a by a less development of one axis, which 

 is the main axis of the body, and is probably usually vertical ; its two poles are always 

 equal. The Discoid lattice-shell is either a biconvex lens (\\dth a thin margin), or a plane 

 disc (a shortened cylinder with thick margin), or some form intermediate between the 

 two. All Discoid shells show a horizontal median plane or equatorial plane, by which 

 they are divided into two equal halves, an upper and lower ; the margin of the lens 

 itself is originally the equator. The main axis, the shortest of all the axes of the shell, 

 stands vertically in the centre of the equatorial plane. Among the Ph^eodauia Discoid 

 shells rarely occur [Aulophacus) , as also among the Acanthaeia (Hexalaspida). 



122. The Larcoid Skeleton or Lentelliptical Lattice-Shell. — The lentelliptical lattice- 

 shells, which may be shortly designated " Larcoid," are especially characteristic of the 

 Larcoidea, a large order of Spumellaria (pp. 599-715; Pis. 9, 10, 49, 50). In 

 addition they recur among the Acanthaeia, in the small family Hexalaspida (p. 872, 

 PI. 139), and the family Diploconida (p. 881, PI. 140), which is derived from it. 

 These lentelliptical lattice-shells are all characterised by the clear differentiation of three 

 unequal, but isopolar dimensive axes, i.e., the three geometrical axes, perpendicular to 

 one another, which determine the form of the shell, are of unequal length ; the two poles 

 of each are, however, equal. The geometrical ground-form is, therefore, a triaxial 

 ellipsoid (§ 34). In the rich order Larcoidea the lentelliptical lattice-shell shows 

 many variations in its development. 



123. The Cyrtoid Skeleton. — Cyrtoid skeletons are those lattice-shells which possess 

 a vertical main axis with two different poles (Monaxonia allopola) ; the upper pole is 

 usually termed the apical, the lower the basal. Such Cyrtoid shells are characteristic of 

 the great majority of the Nassellaeia or MonopyleA (and especially of the Cy rt el- 

 la r i a) ; they are also found in a large division of the Ph^odaeia (the P h 8e o- 

 g r o m i a), and in some Spumellaria. In general the manifold Cyrtoid shells may be 

 divided into two large groups, those with one and those with several chambers. The 

 monothalamous CjTtoid shells are usually ovoid, conical, cap- or helmet-shaped ; their 



(ZOOL. CHALL. EXP. PART XL. — 1886.) Er I 



