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



15. Octopyle decastyle, n. sp. (PI. 9, fig. 13). 



Cortical shell thorny, nearly quadrangular, with ten stronger radial spines, two opposite on the 

 poles of the principal axis (as prolongations of the axial beams), eight others opposite in pairs in 

 two diagonal planes (as in the foregoing species). Lateral gii-dle nearly square, with slightly convex 

 lateral faces, slightly concave principal faces. Transverse girdle broad, with four pores on the 

 isthmus. Sagittal septum of the same length as the hexagonal medullary shell. Eight gates 

 elliptical or irregular roundish. 



Bime-nsions. — Length of the medullary shell 0'05, breadth 0'32 ; diameter of the cortical shell 

 0-15. 



Habitat. — Western Tropical Pacific, Station 224, depth 1850 fathoms. 



Genus 285. Pylonium^ Haeckel, 1881, Prodromus, p. 464. 



Definition. — P y 1 o n i d a with trizonal lentelliptical medullary shell surrounded 

 by three crossed latticed cortical girdles, one smaller (primary) transverse, one larger 

 (secondary) lateral, and one (tertiary) sagittal girdle. 



The genus Pylonium is the most highly developed among the Diplozonaria, with 

 six complete latticed girdles, three on the medullary shell, three on the cortical shell, 

 the latter corresponding to the former. In each of the two systems the transverse gndle 

 is smaller than the lateral ; the sagittal girdle may be smaller or larger than the lateral 

 gndle. Pylonium repeats the typical form of Tvizon ium ; but whilst in this latter 

 the medullary centre of the shell is a simple spherical chamber, here in Pylonium it 

 is a true trizonal or ZarwaciY^a -shaped medullary shell. 



Subgenus 1. Pylonissa, Haeckel. 



Definition. — Cortical shell smooth or thorny, but without large, symmetrically 

 disposed spines. 



1. Pylonium circozonium, n. sp. 



Cortical shell thorny, without radial spines ; all its three girdles nearly of the same size, 

 subcircular, very narrow, only with one to two rows of pores. Four gates subcircular. 



Dimensions. — Principal axis 0'12, transverse axis O'll ; diameter of the subspherical medullary 

 shell 0-05. 



Hahitat. — North Pacific, Station 244, depth 2900 fathoms. 



' Pi/Zo)u'it7?i= Building with gates ; icvT^ayiou. 



