REPOET ON THE RADIOLARIA. 



'27 



Tlie Acauthouida, the second suborder of Acanthometra, embraces by far 

 the greatest number in this order, viz., all those forms in which twenty radial spines are 

 regularly disposed after the MiiUeriau law — Icosacantha (compare above, p. 717). The 

 radial spines of this suborder are either simple or provided with transverse processes 

 (either two opposite or four crossed apophyses). They are commonly united in the 

 middle of the central capsule by their opposed basal ends, forming small pyramids ; the 

 meeting triangular faces of the neighljouriug pyramids being propped one upon another. 

 Above these small basal pyramids often arises a basal leaf-cross formed by four broad 

 triangular leaves or wings with straight edges ; the meeting thin edges of the neighbour- 

 ing spines serve for strengthening the basal junction and form hollow pyramidal spaces 

 or compartments, filled with the contents of the central capsule (compare p. 721). 

 The suborder A c a n t h o n i d a comprises three different families, the Astrolonchida, 

 Quadrilonchida, and Amphilonchida. The first family, the Astrolonchida, comprises by far 

 the greater number of the Acanthonida; those genera in which all twenty spines are 

 perfectly equal or nearly equal in size and form. In the second familj^ the Quadrilon- 

 chida, the four equatorial spines are much larger (and often also of another form) than 

 the sixteen other spines (often also the eight tropical larger than the eight polar spines). 

 The third family, the Amphilonchida, is distinguished by the preponderating development 

 of only two opposite ecpiatorial spines, which are much larger (and often also of another 

 form) than the eighteen other spines. 



Synojosis of the Sitborders and Families of A c a n t h o m e t r a. 



Suborder I. ACTINELIDA. 



Number of tlie radial spines variable, 



either more or less than twenty, 



commonly disposed irregularly and 



not according to the MiiUerian law. 



Eadial spines very numerous (thirty to a hundred 

 or more), radiating from a common centre 

 ■within a spherical space, . . .1. Asteolophida. 



Radial spines between ten and twenty, radiat- 

 ing from one common point within a sphere- 

 quadrant, . . . . .2. LlTHOLOPHIDA. 



Eadial spines of variable number ; every two 

 opposite spines grown togetlier in the centre ; 

 therefore numerous diametral spines are 

 crossed freely in the centre, . . 3. Chiastolida. 



All twenty radial spines nearly equal, and of 



the same size and form, . . .4. AsTROLONcniDA. 



Suborder II. ACANTHONIDA. 



Number of the radial spines constantly j 

 twenty, disposed regularly accord 

 ins to the Miillerian law. 



Four equatorial spines much larger than (and 

 often also of different form from) the sixteen 

 other sjiines, .... 



Two opposite equatorial spines (or principal 

 spines) much larger than (and often also of 

 different form from) the eighteen other 

 spines, ..... 



5. Quadrilonchida. 



6. Amphilonchida 



