REPORT ON THE RADIOLARIA. 



727 



The Acanthonida, 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 Miillerian 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 neighbouring 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 Acanthonida 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 family, 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 equatorial spines, which are much larger (and often also of another 

 form) than the eighteen other spines. 



Synopsis of the Suborders and Families of Acanthometra. 



Suborder I. ACTINELIDA. 



Number of the radial spines variable, 

 either more or less than twenty, < 

 commonly disposed irregularly and 

 not according to the Miillerian law. 



Suborder II. ACANTHONIDA. 

 Numberof the radialspines constantly j 

 twenty, disposed regularly accord- 

 ing to the Miillerian law. 



Radialspines very numerous (thirty toahundred 

 or more), radiating from a common centre 

 within a spherical space, . 



Radial spines between ten and twenty, radiat- 

 ing from one common point within a sphere- 

 quadrant, ..... 



1. ASTROLOPHIDA. 



2. LlTHOLOPHIDA. 



Radial spines of variable number; every two 

 opposite spines grown together 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, 



Four equatorial spines much larger than (and 

 often also of different form from) the sixteen 

 other spines, .... 



Two opposite equatorial spines (or principal 

 spines) much larger than (and often also of 

 different form from) the eighteen other 

 spines, ..... 



4. ASTROLONCHIDA. 



5. QUADRILONCHIDA. 



6. AMPHILONCHIDA 



