(See Gener. Morphol., Bd. i. pp. 459-874.) {xvii}36. _The Amphithect Pyramidal Ground-Forms._--The ground-forms whose geometrical type is the amphithect pyramid, are distinguished from the regular pyramidal forms, just discussed, chiefly by the form of the basal plane, which is not a regular, but an amphithect or elongated polygon (in the simplest case a rhombus). Hence in this case the allopolar main axis of the body is crossed by two transverse axes which are isopolar and at right angles, but are unequal; they cannot, however be distinguished as sagittal and frontal axes as is the case in the zeugites. In the animal as well as in the vegetable kingdom, an important part is played by this ground-form, _e.g._, in the Ctenophora, where it is the rhombic pyramid. Among the Radiolaria it is not common, though it is clearly expressed among the NASSELLARIA in a number of #Stephoidea# (Stephanida and Tympanida), as well as in many #Spyroidea# (_e.g._, the bipedal Zygospirida). It is very accurately developed among the PHÆODARIA in the bivalved #Phæoconchia# (Pls. 121-128), where the two valves of the shell (dorsal and ventral) are generally exactly alike, their median keels corresponding to the poles of the sagittal axis. In the slit between the two valves lie the two secondary openings (right and left) of the tripylean central capsule, corresponding to the two poles of the frontal axis, and the main axis stands perpendicularly to both these, its oral pole being indicated by the astropyle, or principal aperture. (See Gener. Morphol., Bd. i. pp. 479-494.)