688 THE VOYAGE OF H.M.S. CHALLENGER. 



lying opposite in pairs in two crossed diagonal planes. Therefore the number of the 

 septa (four) and the domes (eight) is here doubled. Among the eight cupolas we 

 distinguish four median (on both sides of the sagittal plane) and four lateral (on both 

 sides of the equatorial plane) ; both groups are of different size and form. Probably 

 Zonidium is derived from Zonarium by lattice-connection between the eight diagonal 

 wing-spines, which in both species of this genus are present, the same as in Tetrapyle 

 octacantha. 



1. Zonidium octostylium, n. sp. 



Cortical shell nearly quadrangular, with rounded corners and thorny surface. Eight long and 

 thin radial spines on both sides of the lateral plane opposite in pairs and lying in two crossed 

 diagonal planes (as in Tetrapyle octacantha). Sagittal constriction three times as long as the lentel- 

 liptical medullary shell. Four medial cupolas (on both sides of the sagittal plane) somewhat 

 larger than the four lateral cupolas (on both sides of the equatorial plane). 



Dimensions. Length of the cortical shell 015, breadth 012 ; length of the medullary shell 

 0-05, breadth 0'03. 



Habitat. Indian Ocean, Madagascar, Eabbe, surface. 



2. Zonidium octotholium, n. sp. (PL 50, fig. 12). 



Cortical shell nearly octangular, with spiny surface ; twenty long and stout radial spines 

 between numerous smaller spines ; eight wing-spines opposite in pairs in two crossed diagonal 

 planes (as in the preceding species) ; twelve other strong spines in the lateral plane (four longer 

 opposite in pairs on the poles of the principal and transverse axes, eight others smaller, alternating 

 between these and the diagonal spines). Sagittal constriction twice as long as the hexagonal medullary 

 shell. Four median cupolas somewhat smaller than the four lateral cupolas. 



Dimensions. Length of the cortical shell 018, breadth 015 ; length of the medullary shell 

 0-07, breadth 0'05. 



Habitat. Indian Ocean, Zanzibar, Pullen, depth 2200 fathoms. 



Family XXIX. LITHELIDA, Haeckel (PI. 49, figs. 1-7). 



Lithelida, Haeckel, 1862, Monogr. d. Eadiol., p. 515. 



Definition. L arcoidea with symmetrical spiral shell, divided by the spiral plane 

 into two symmetrical halves ; all windings of the spiral lie in this plane. Primordial 

 chamber either simple or Zarnaa'^a-shaped. 



The family Lithelida comprises all those Larcoidea in which the growth of 

 the latticed shell is spirally winding in one plane, Nautilus-like. They agree in the spiral 

 growth with the following family. But in the Streblonida the spiral is screw-shaped, 



