REPORT ON THE RADIOLARIA. 661 



particularly than in the nearly allied Pylonida; the greater number of the species are very 

 rare. Besides this they seem to be very variable and inclined to produce numerous 

 abnormalities. Very often intermediate forms of transition are to be found between the 

 Tholonida and other Larcoidea, particularly the Pylonida and Lithelida. In many 

 species the thick-walled shell is very opaque, and offers great difficulties to the clear study; 

 in the greater number the structure of the shell cannot be understood completely without 

 rolling the shell to the different sides ; and the distinction between the Staurotholida and 

 Cubotholida is often very difficult. 



The primordial chamber of the Tholonida, or the central chamber in which its growth 

 begins, is either a simple lentelliptical shell (without enclosed medullary shell), like 

 Cenolarcus, or it is a trizonal shell, like Larnacilla, and contains a small concentric 

 medullary shell. But this important inner shell of the central chamber offers peculiar 

 difficulties for study. In many cases (probably in the greater number) there is no doubt 

 the same characteristic trizonal medullary shell, which we found in the Larnacida and 

 Pylonida, and this is our principal argument, if we regard the Tholonida as Larcoidea, 

 which are most nearly allied to both these families, and in which the fenestrated open 

 cortical girdles of the Pylonida are replaced by fenestrated closed cupolas or domes ; the 

 characteristic "gates" or large fissures in the cortical shell of the former are therefore 

 here perfectly closed by network. The Tholonida agree in this point with the Larnacida, 

 but are distinguished from them by the prominent vaultings of the hemispherical 

 cupolas or domes, which give them a peculiar appearance. Each pair of domes (opposite 

 on both poles of" one dimensive axis) corresponds to one single girdle of the 

 Pylonida. 



Regarding the absence or presence of a medullary shell in the central chamber, we 

 may divide the Tholonida into two groups, Cenotholida (without medullary shell) and 

 Coccotholida (with medullary shell). The absence of the medullary shell in many 

 Tholonida may be either primary (original) or secondary (by reduction and loss of it). 

 Probably in the larger proportion (if not always ?) its absence is the consequence of reduc- 

 tion and loss, and in this case the Cenotholida must be derived phylogenetically from the 

 Coccotholida, but possibly often (or always ?) the contrary may also be the case. At 

 present we cannot find certain arguments for one opinion or the other. 



The connection between the medullary (internal) and the cortical (external) shell 

 of the central chamber in the Coccotholida is effected by two lateral wings of a 

 latticed transverse girdle ; on both sides of the latter remain the four internal open 

 " gates " of the Pylonida and Larnacida. Very often the limiting edges of these four 

 gates are prolonged into eight external radial spines which lie opposite in pairs in two 

 crossed diagonal planes, and correspond to the eight portal-spines of Tetrapyle octacantha. 

 Besides these, we often find four other beams opposite in pairs in the two crossed axes 

 (principal and lateral). Very often also these radial beams (between medullary and 



