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



2. Pylozonium octacanthum, n. sp. (PL 9, fig. 16). 



Outer cortical shell lentelliptical, one and a third times as long as broad, with thorny surface 

 and eight long and thin radial spines arising from the corners of the four elliptical gates, and 

 lying opposite in pairs in two crossed diagonal planes. Inner cortical shell of the same shape but 

 one-fourth smaller, about twice as large as the lentelliptical medullary shell. 



Dimensions. Length of the medullary shell O065, breadth 0'045 ; length of the inner cortical 

 shell 018, breadth 0'12 ; length of the outer cortical shell 0'24, breadth 018. 



Habitat. North Pacific, Station 244, depth 2900 fathoms. 



Family XXVII. THOLO NID A, n. fern. (PI. 10). 



Definition. L arcoidea with regular, completely latticed cortical shell, which is 

 composed of two to six or more hemispherical or cap-shaped domes (vaulted chambers or 

 cupolas). The domes lie opposite in pairs on the poles of the three dimensive axes, are 

 separated by annular constrictions, and surround a simple or Larnacilla-shajped central 

 chamber. 



The family Tholonida represents a peculiar and very remarkable group of the 

 Larcoidea, distinguished from the other groups of this suborder by the characteristic 

 form of the shell, composed of a variable number of hemispherical domes or cupolas. The 

 middle and original part of the shell is constantly formed of an elliptical or subspherical 

 central chamber, which often, but not constantly, contains a small medullary shell. An 

 even number (two, four, six, or more) of domes is attached to the poles of the three 

 dimensive axes of the central chamber ; according as only one, or two, or all three axes 

 develop cupolas, we distinguish in this subfamily three different subfamilies (the 

 Amphitholida, Staurotholida, and Cubotholida). The Amphitholida (or Tholonida 

 monaxonia) form cupolas only on the two poles of one single axis, and this axis 

 corresponds to the minor (or transverse) axis of the central chamber, we find here 

 therefore constantly at least two lateral cupolas (PI. 10, figs. 1-7). The Staurotholida 

 (or Tholonida diaxonia) form cupolas on the poles of two axes perpendicular one to 

 another; these two axes are the major (principal) and the minor (transverse) axis of the 

 central chamber, we find here therefore constantly at least four cupolas crossed in pairs 

 (PI. 10, figs. 8-11). The Cubotholida (or Tholonida triaxonia) form cupolas on the 

 poles of all three dimensive axes (perpendicular one to another); corresponding to the 

 principal, transverse, and sagittal axes of the lentelliptical central chamber ; therefore we 

 find here constantly at least six cupolas, attached in pairs on the six sides of the central 

 (Chamber (PL 10, figs. 12-17). 



The number of genera and species in the family Tholonida is rather large, but the 

 number of individuals is much smaller than in most of the other SPTJMELLARIA, and 



