REPORT ON THE RADIOLARIA. xvii 



36. The Amphithect Pyramidal Ground-Forms. — The ground-forms whose geometrical 

 type is the amphithect pyramid, are distinguished from the regular pj'ramidal 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 rhomlius). Hence in this case 

 the alloplar 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 Eadiolaria it is not common, though it is 

 clearly expressed among the Nassellaria in a number of Stephoidea (Stephauida 

 and Tympanida), as well as in many Spyroidea {e.g., the bipedal Zygospirida). It 

 is very accurately developed among the Ph^odaria in the bivalved Phaeoconchia 

 (Pis. 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.) 



37. The Amphipleural Ground-Forms. — By the term amphipleural ground-forms are 

 to be understood those usually defined as " bilaterally radial"; their geometrical type is a 

 half amphithect pyramid. The best known examples of this form in the animal kingdom 

 are the bilateral five-rayed Echinoderms (Spatangus, Clypeaster), in the vegetable 

 kingdom the symmetrical five-rayed flowers ( Viola, Trifolium). The three dimensive 

 axes have the same relation as in the zygopleura, to be next discussed, and which also 

 resemble them in being divisible only by one plane (the sagittal median plane) into two 

 equal halves. They differ, however, the amphipleural body not being made up of two 

 antimeres, but of at least three pairs of antimeres (or three parameres), being therefore 

 primitively radial. Hence each of the symmetrical halves of the body contains more 

 than one antimere. Among the Eadiolaria this form does not occur in the Spumellaria, 

 AcANTHARiA, or Ph^odaria ; it is very common, however, among the Nassellaria ; 

 many Cyrtoidea multiradiata and Spyroidea multiradiata show this bilaterally 

 radial ground-form, inasmuch as the body consists of two symmetrical halves, and is also 

 composed of numerous (usually three, six, nine, or more) radial parameres. In the multi- 

 radiate Dicyrtida and Tricyrtida the cephahs (the first joint) is usually bilateral, whilst the 

 thorax (the second joint) is multiradial. (See Gener. Morphol., Bd. i. pp. 495-506.) 



38. The Zygopleural Ground-Forms. — As zygopleural or dipleural ground-forms, as 

 opposed to the amphipleural, are classed those zeugites or centroplana which are known 



(ZOOL. CHALL. EXP. — PART XL. 1880.) . ■ Rf C 



