REPORT ON THE RADIOLARIA. CXI 



from a study of the concentrically disposed lattice-shells of some Larcoidea [Cocco- 

 larcus, Larcidium, PI. 50, fig. 8), in which the inner medullary shell is spherical, the 

 outer cortical shell more or less elliptical In the great majority of Larcoidea the 

 latter arises in quite a peculiar manner, three broad lattice-zones, which are developed in 

 three planes at right angles to each other, growing out from a small spherical or 

 lenteUiptical medullary shell, Trizonium, Larnacilla (compare pp. 600, 615, 628, &c.). 



The trizonal Larnacilla-sheW commences by the formation of a transverse girdle, by the union 

 of two lateral latticed processes, which spring right and left in the equatorial plane from the poles 

 of the frontal axis of a lenteUiptical medullary shell {Momzonium, p. 633, PI. 9, fig. 1). This is 

 followed by a second lateral girdle, which lies in the frontal plane and proceeds from its lateral 

 poles (Bizonium, p. 634, PI. 9, figs. 2, 3). Finally the sagittal girdle is formed, lying in the sagittal 

 plane and arising from the lateral girdle on the two poles of the main axis {Trizonium, p. 637, 

 PI. 9, fig. 4). Whilst the gaps between the three zones of this trizonal shell remain open in the 

 Pylonida, in Larnacilla, the important primitive form of the Larnacida, they are closed by lattice- 

 work (PI. 50, figs. 3-8). From this trizonal Larnacilla-shell the great majority of Larcoid shells 

 may be derived. Such a system of zones may be repeated (Diplozonaria) or even developed a 

 third time (Triplozonaria, p. 632). In most Larcoidea the zones are secondarily connected by 

 lattice-work. In the Tholonida (PI. 10) each of the two opposite latticed wings of a zone becomes 

 a closed dome. In the Zonarida (PI. 50 figs. 9-12) these domes are partially or wholly bisected 

 by constrictions or latticed septa which are developed in the three dunensive planes. The 

 Lithelida (PI. 49, figs. 1-7) are characterised by the fact that one of each pair of opposite latticed 

 processes (or half zones) grows more strongly than the other, and that the larger completely 

 embraces the smaller so as to form a complicated spiral. "Whilst in this case the spiral lies in a 

 plane, in the Streblonida (PI. 49, figs. 8, 9) it becomes turbinoid like a gastropod shell and forms an 

 ascending spiral. Finally, two small families of Larcoidea are characterised by quite irregular 

 growth (a very rare occurrence among the Eadiolaria) ; these are the simple-chambered Phorticida 

 (PL 49, figs. 10, 11), and the many chambered Soreumida (PI. 49, figs. 12, 13). The phylogenetic 

 relationship of these families of Larcoidea is probably very complicated and demands closer 

 investigation (compare pp. 599—604). 



168. Descent of the Polycyttaria. — The polyzootic or colonial Eadiolaria, which we 

 unite in the group Polycyttaria (sometimes known as " Sphserozoea"), belong without 

 doubt to the legion Spumellaria, for they possess all the peculiarities by which these 

 Peripylea are distinguished from the other legions of the Eadiolaria. Only the 

 morphological position of the Polycyttaria in that legion, and their phylogenetic relation 

 to the monozootic or solitary Spumellaria, can be variously interpreted. The three 

 families which we distinguish among the Polycyttaria are so closely related to three 

 different families of the Monocyttaria, that they may be directly derived from them by 

 the formation of colonies. According to this triph^jletic hypothesis the social skeletonless 

 Collozoida (PI. 3) would be descended from the solitary ThalassicoUida (PI. 1), the 

 polyzootic Sphserozoida with a Beloid skeleton (PI. 4) from the monozootic Thalasso- 



