xvi THE VOYAGE OF H.M.S. CHALLENGEK. 



the four equatorial spines are of equal dimensions (Icosacantha). (See Gener. Morpliol., 

 Bd. i. p. 436-446.) 



34. Tlie Amphithect Dipyramidal or LenteUiptical Ground Forms. — The ground- 

 forms whose geometrical type is the lenticular or "triaxial" ellipsoid, may also be 

 designated amphithect double pyramids ; they are characterised by the possession of a 

 vertical main axis which has similar poles, and is crossed at its middle by two transverse 

 axes, unequal but isopolar. The horizontal equatorial plane of the body is therefore an 

 amphithect or elongated polygon (a rhombus in the simplest case possible), and divides 

 the whole body into two equal amphithect pyramids. The simplest and commonest 

 form of this group is the rhombic octahedi-on, which is also the ground-form of the 

 rhombic crystallographic system. It plays an important part in those Acanthakia in 

 which twenty radial spines are disposed according to the Miillerian Law, but in which the 

 two pairs of equatorial spines are unequal (different geotomical and hydrotomical axes, 

 see p. 719); to this category belong the Amphdonchida (PL 132), Belonaspida (PL 136), 

 Hexalaspida (PL 139), and Diploconida (PL 140). A form essentially identical obtains 

 also among the Spumellaria in the majority of the Larcoidea, both in their triaxial 

 lattice-shells, and in their lenteUiptical central capsules, which present geometrically 

 accurate triaxial ellipsoids, with three unequal isopolar axes at right angles to each other. 

 (See Gener. MorphoL, Bd. i. p. 446-452.) 



35. The Eegidar Pyramidal Ground-Forms.— The ground-forms whose geometrical 

 type is the regular pyramid, and which are the most conspicuous in the Medusae, Polyps, 

 Corals, and regular Echinoderms (the Radiata of earlier authors), are almost confined 

 among the Eadiolaria to the legion Nassellaria; they occur, however, in the great 

 majority of these, and especially in those families which may be classed together as 

 "Cyrtoidea triradiata et multiradiata." Strictly speaking, however, almost aU these 

 Nassellaria, at aU events in theu- origin, are bilateral or dipleuric, since the primary 

 sagittal ring with its characteristic apoj^hyses marks out the sagittal median plane, and 

 further, since the three feet of the basal tripod are usually divided into an unpaired dorsal 

 (pes caudalis) and two paired ventral or lateral (pedes pectorales, dexter et sinister). 

 On the other hand, it is noteworthy, firstly, that among the primitive Plectoidea 

 there are perfectly regular radial forms, without any indication of an original bilateral 

 symmetry, and secondly, that similar forms are also very common among the 

 Cyrtoidea, probably as secondary radial forms, developed from primitive bilateral 

 ones. Similar cases also occur in certain Ph^odaria {e.g., the Medusettida and 

 Tuscarorida, Pis. 100, 120), but they are entirely wanting among the Acantharia and 

 Spumellaria. The multiradial Nassellaria have arisen from the triradial by the 

 interpolation of three, six, nine, or more interradial and adradial secondary apophyses 

 between the three primary perradial ones. (See Gener. MorphoL, Bd. i. pp. 459-874.) 



