REPORT ON THE RADIOLARIA. 933 



three different dimensive axes : the principal axis with different apical aad basal poles, 

 the sagittal axis with different dorsal and ventral poles, and the lateral axis with equivalent 

 right and left poles. In only a few genera this bilateral symmetry is not expressed, and 

 a simpler, more regular fundamental form appears. The latter may be either primary 

 (in the monaxonian Archicircus and Lithocircus) or secondary, afterwards acquired (in 

 the octahedral Trissocircus and Trissocydus, the cubical Lithocubus, the prismatic 

 Eutympanium, and some other forms). 



The most important element of the skeleton, with which its formation begins, in 

 all Stephoidea is the simple primary or sagittal ring, lying vertically in the 

 sagittal or median plane of the body and surrounding the monaxonian central capsule. 

 This sagittal ring is the only essential element of the skeleton in all Stephanida, and is 

 completely preserved in all Semantida, also in the greater part of the Coronida and 

 Tympanida. It is partially reduced in the small groups of the true Acanthodesmida 

 (subfamily of Coronida) and the Dystympanida and Eutympanida (subfamilies of 

 Tympanida). Here only the vertical parts of it are preserved (dorsal and ventral rod), 

 whilst the horizontal parts are lost (mitral and basal rod). 



The sagittal ring, lies constantly in the vertical median plane of the body, and. 

 therefore divides the enclosed central capsule into a right and a left half. It is rarely 

 regular or subregular, commonly dipleuric or distinctly bilateral, so that we may easily 

 distinguish its dorsal and ventral, apical and basal parts. The most important of these 

 four parts or " rods " is the " basal rod " or the inferior part, because here the ring is in 

 closer connection with the central capsule and its "porous area"; here peculiar spines 

 or branches are commonly developed, which even on the isolated ring immediately 

 determine the basal pole. The opposite upper part, or the " mitral rod," is also often 

 distinguished by peculiar appendages. The posterior part, or the " dorsal rod " (the 

 anterior rod, a, in the description of Biitschli), is commonly more straight, often quite 

 vertical. The opposite anterior part, or the " ventral rod " (the posterior rod, &, of 

 Biitschli), is usually more convex, and often strongly curved or semicircular, 



AVhilst this dipleuric or bilateral (commonly obliquely ovate or nearly triangular) form 

 of the sagittal ring is distinctly preserved in by far the greater number of Stephoidea, 

 it is replaced in some few genera by a more regular, amphitheet, diphragmatic, or biradial 

 form. In this case we may often suppose a primary regularity to exist, the dorsal and 

 ventral parts being not yet differentiated, as in Archicircus and Lithocircus, Zygo- 

 stephanus and Protympanium, and perhaps also in some other forms. But in other cases 

 the regularity is, on the contrary, secondary, being derived from original bilateral forms. 



The rod of the sagittal ring is either cylindrical (with circular transverse section) or 

 angular (commonly with triangular transverse section). In nearly all Stephoidea 

 (with very few exceptions) branches or apophyses are developed from the ring, 

 regular^ disposed and often of very great morphological importance. Commonly "these 



