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



between the two foregoing genera, and has six caipolas, three on each side of the sagittal 

 septum. 



The inner communication of the cupolas or chambers is more or less free, the lattice- 

 work of the separating septa between them commonly remaining more or less imperfect, 

 or represented only by some isolated beams or meshes. The outer network of the cupolas 

 is commonly irregular (as in the majority of Larcoidea), but sometimes distinguished 

 by a small number of regularly disposed larger apertures (similar to the " gates " of the 

 Pylonida). From the surface radial spines often arise in characteristic number and 

 symmetrical disposition, commonly as prolongations of the septal axes or of the con- 

 stricted edges. 



The Medullary Shell in all Zonarida is a true trizonal or Zarnact7?a-shaped lattice- 

 shell (compare above, p. 600) ; its perimeter (or the first lateral girdle) is sometimes more 

 elliptical, at other times more hexagonal ; the hexagon is amphithect ; both its lateral 

 sides are often concave and commonly longer than the four other sides. 



The Central Capsule in all Zonarida is in a strict geometrical sense a true lentel- 

 lipsis (compare above, p. 599) ; its principal axis is commonly one and a third to one and 

 a half times as great as the transverse axis, and twice to three times as great as the 

 sagittal axis. The lentelliptical central capsule encloses the trizonal medullary shell, 

 whilst it is externally enveloped by the chambered cortical shell. 



Synojisis of the Genera of Zonarida. 



Two annular constrictions and fonr cupolas, .... 301. Zonariiim. 



Three annular constrictions and six cupolas, .... 302. Zoniscus. 



Four annular constrictions and eight cupolas, .... 303. Zonidium. 



Genus 301. Zonarium^ n. gen. 



Definition. — Z o n a r i d a with four dome-shaped chambers of the cortical shell, 

 separated by two annular constrictions (one sagittal and one transverse). 



The genus Zonarium is the most simple form of the Zonarida, and differs from the 

 nearly allied Larna^ealpis by two ring-like constrictions, which are crossed at right 

 angles, one in the sagittal (or median) plane, and one in the transverse (or ecjuatorial) 

 plane. By these two annular constrictions four egg-shaped or kidney-shaped chambers 

 become imperfectly separated, which correspond to the quadi'ants of the lateral 

 plane. The first cause of the marked constrictions may be the formation of the latticed 

 sagittal septum, which is found first in Octopyle, as halving the four gates of 



' Z(niri'!tm = Small girtlle ; ^ana^iov. 



