REPORT ON THE RADIOLARIA. 703 



family, but whilst in the Lithelida the spiral line lies in one plane (as in Nautilus), 

 in the Streblonida it ascends like a screw (as in Helix). Therefore the former have 

 the same relation, regarding the spiral structure, to the nautiloid Polythalamia as the 

 latter to the turbinoid Foraminifera. Indeed the single forms of Streblemida repeat in 

 their special structure the characteristic genera of Turbinoida, such as Globigerina, 

 Rosalind, Pulvinulina, Hastigerina, &c. As in these calcareous turbinoid Foraminifera, 

 so also in the analogous siliceous Streblonida the distinction of species is very difficult and 

 open to many objections. 



The number of species in this family is very small ; all are rare and for the most part 

 very opaque and difficult to understand, so that the following distinction of a dozen 

 species can have only a provisional value. To get a complete idea of their peculiar 

 structure, the shell must be turned and observed from different sides, and thus their full 

 study requires yet much time and work. There are to be found evident transitional 

 forms between them and the Lithelida on the one hand and the Soreumida on the other. 

 Besides this, most species of Streblonida seem to have more inclination to individual 

 varieties and abnormalities than the majority of the other Radiolaria. 



The general form of the whole shell is in the Streblonida sometimes more egg-shaped 

 or even subspherical, at other times more top-like or conical, sometimes nearly discoidal. 

 The height of the shell (or the vertical axis of the ascending spiral) is occasionally larger, 

 at other times smaller than the breadth (or the greatest horizontal diameter, perpendicular 

 to the height). Some very flat forms seem to approach the Lithelida. With regard to 

 the internal screw-formation, the shell of all Streblonida is asymmetrical. 



The number of the aggregated incomplete chambers is commonly between ten and 

 twenty, but ascends sometimes to thirty, forty, or more. Sometimes the size of all the 

 chambers is nearly the same, sometimes they increase gradually, occasionally also very 

 rapidly. The primordial chamber (or the first and oldest) seems to be commonly the 

 smallest, and inversely, the last and youngest chamber, the largest. But sometimes (in 

 Streblopyle) also the contrary may be the case. The form of the single chambers is very 

 variable, from the spherical or hemispherical through all transitions leading to irregular 

 roundish or longish forms. The network is commonly irregular, with small roundish 

 pores of different sizes, but sometimes also regular, circular. The surface of the shell is 

 commonly smooth or rough, rarely covered with radial spines. In most species the 

 reticulation and particularly the separation of the chambers is more or less incomplete. 



As in the Lithelida, so also in the Streblonida we can distinguish two subfamilies. 

 In the Streblacanthida (Streblonia, Streblacantha) the primordial chamber is a simple, 

 spherical, subspherical, or lentelliptical latticed shell. In the Streblopylida (Streblopyle) 

 the primordial chamber is trizonal or Larnacilla-sliaped, as in the greater number of 

 Larcoidea, composed of three elliptical latticed girdles of "unequal size, perpendicular 

 one to another, and surrounding a simple central chamber. As in the Lithelida, so also 



