658 THE VOYAGE OF H.M.S. CHALLENGEE. 



The genus Tetrapylonium represents a further developmental stage of the foregoing 

 Amphipylonium ; whilst in this latter the outer cortical shell (or the third system of 

 girdles) is formed only by a transverse girdle, here this is crossed by a lateral girdle. 

 Tetrapylonium repeats therefore the typical form of Tetrapyle, but with doubled 

 cortical shell (Prodromus, 1881, p. 464). 



1. Tetrapylonium pantellipticum, n. sp. 



Outer cortical shell elliptical, one and a third times as long as broad, with smooth surface and 

 four elliptical gates. Inner cortical shell elliptical, with smooth surface and four elliptical gates 

 which repeat the form of the four outer gates, but are of half the size. Transverse girdle 

 broad, with five pores on the isthmus. 



Dimensions. Length (or principal axis) of the first shell (medullary shell) 0'04, breadth (or 

 transverse axis) 0'03 ; length of the second (or inner cortical) shell 012, breadth - 09 ; length of 

 the third (or outer cortical) shell 018, breadth 014. 



Habitat. Pacific, central area, Station 274, depth 2750 fathoms. 



2. Tetrapylonium reniforme, n. sp. 



Outer cortical shell elliptical, one and a half times as long as broad, with thorny surface and 

 four large kidney-shaped gates. Inner cortical shell of the same form, but one-third smaller, 

 also with four kidney-shaped gates. Transverse girdle small, but with two pores on the 

 isthmus. 



Dimensions. Length of the first (innermost) shell 0'03, breadth 0'02 ; length of the second 

 (middle) shell 014, breadth 0'08 ; length of the third (outermost) shell 0"2, breadth Oil. 



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



3. Tetrapylonium quadrangulare, n. sp. (PL 9, fig. 15). 



Outer cortical shell nearly quadrangular, one and a .third times as long as broad, with four 

 rounded corners, from which arise in the lateral plane four three-sided pyramidal radial spines, 

 opposite in pairs in two crossed diagonal axes. Surface thorny ; four gates elliptical or nearly 

 quadrangular. Inner cortical shell more elliptical, by one half smaller, with broader transverse 

 girdle and four kidney-shaped gates. Medullary shell nearly spherical, of half the size. 



Dimensions. Length of the first shell 0'045, breadth 0'035 ; length of the second shell 01, 

 breadth 0'07 ; length of the third shell 016, breadth Oil. 



Habitat. South Atlantic, off Tristan da Cunha, Station 332, depth 2200 fathoms. 



4. Tetrapylonium octacanthum, n. sp. 



Outer cortical shell elliptical, 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 triangular gates, and lying 



