REPORT ON THE RADIOLARIA. 



691 



thin spiral lamellae and give to the whole shell more solidity. Often these beams form 

 imperfect radial septa, by which the spiral cavity of the turnings is divided into a variable 

 number of chambers. But these chambers never become so regular and perfect as in the 

 analogous nautiloid Polythalamia, 



In many Lithelida the growth of the shell reaches a certain limit, concluding with 

 the formation of a superficial latticed lamella of lentelliptical or nearly spherical form. 

 In many other forms of the family this seems not to be the case ; but these may possibly 

 be younger forms, afterwards reaching the same limit. 



The network of the shell in the Lithelida is commonly quite irregular, and so variable 

 that its special conformation has usually no value in the determination of the species. 

 The surface of the shell is often covered with radial spines, which are sometimes 

 arborescent. 



The central capsule seems always to preserve the same lentelliptical form (or triaxial 

 ellipsoid) as in all other Larcoidea. With the increase of growth it encloses succes- 

 sively a larger part of the spiral cortical shell, but on the outside is constantly protected 

 by the last turnings of the spiral, or by the lattice-lamella of the surface. 



Synopsis of the Genera of Ijithelida. 



Surface of the cortical shell smooth or thorny, without 



radial spines, . . . 304. Spirema. 



I. Subfamily 



Spireimda. 

 Central medullary shell simple, Suj , face rf the cortical ghfiU 



spherical or lentelliptical. gimp]e op branched radial spineg) _ 



numeroU3 



305 



II. Subfamily 



Larcospirida. 



Central medullary shell double, 

 trizonal or Larnacilla-shaped. 



Lithelius. 

 Larcospira. 

 Pylospira. 

 The sagittal girdle turns around the transverse axis, . 308. Tholospira. 



The transverse girdle turns around the principal axis, 306. 

 The lateral girdle turns around the sagittal axis, . 307. 



Both wings of the transverse girdle turn around the 



principal axis in an opposite diagonal direction, . 309. Spironium. 



Subfamily 1. SPIREMIDA, Haeckel, 1881, Prodromus, p. 464. 

 Definition. L ithelida with simple, spherical or subspherical, medullary shell. 



Genus 304. Spirema, 1 Haeckel, 1881, Prodromus, p. 464. 



Definition. Lithelida with simple, spherical or subspherical, medullary shell, 

 and lentelliptical or subspherical, spirally constructed cortical shell ; surface smooth or 

 thorny, without radial spines. 



1 Spirema = Convolution, turning ; wil^ 



