1362 THE VOYAGE OF H.M.S. CHALLENGER. 



siibspherical. Abdomen subovate, prolonged into a pyramidal, axial, basal spine, half as long as the 

 thorax. Pores subregular, circular. From the upper half of the abdomen arise three stout, lateral, 

 divergent wings of about the same length, which are curved inwards and fenestrated both at the 

 Ijroad triangular base and at the thickened, three-edged distal end. 



Dimensions. — Length of the three joints, a 0'02, l 0'06, c 0'12 ; breadth, a 0'03, b O'OS, c 0'08. 



Habitat. — Central Pacific, Station 266, depth 2750 fathoms. 



Genus 600. Lithochytris,^ Ehrenberg, 1847, Monatsber. cl. k. preuss. Akacl. 



d, Wiss. Berlin, p. 54. 



Definition. — T heoperida (vel Tricyrtida triradiata clausa) with a three-sided 

 pyramidal abdomen, the triangular base of which is prolonged at the three corners into 

 three terminal feet. 



The genus Lithochytris diflers from all the other Theoperida in the absence of 

 lateral wings and the possession of three divergent, terminal feet, which arise from the 

 three corners of the three-sided pyramidal abdomen. It repeats, therefore, among the 

 Tricyrtida, that characteristic formation which Sethochytris and Tetrahedrina represent 

 among the Dicyrtida. Some species may be derived from Pterocanium, other species 

 from Podocyrtis, by the development of a lattice-plate closing the terminal mouth. 



Subgenus 1. Lithochy trades, Haeckel. 



Three feet of the abdomen solid, not fenestrated, representing external apophyses 

 of the triangular shell-base. 



1. Lithochytris cortina, n. sp. (PL 67, fig. 12). 



Shell three-sided pyramidal, without external strictures, but with two internal girdles. Length 

 of the three joints = 2 : 2 : 5, breadth = 3 : 4 : 6. Cephalis large, truncate-pyramidal, with nine deep 

 divergent ribs (and nine longitudinal rows of pores between them), and with a stout pyramidal horn 

 of half the length. The three prominent edges of the thorax and abdomen are prolonged over the 

 base of the pyramid into three very stout, solid, subovate feet, about as long as the thorax. 



Dimensions. — Length of the three joints, a 0-04, b OM, c 01 ; breadth, a 0-06, b O'OS, c 012. 



Habitat. — Central Pacific, Station 266, depth 2750 fathoms. 



2. Lithochytris pyriformis, n. sp. (PI. 67, fig. 13). 



Shell pyramidal, nearly pear-shaped, with two distinct strictures. Length of the three joints = 

 1:1:3, breadth = 1:2:3. Cephalis large, pear-shaped, with a short pyramidal ,horu of half the 



^■Lithochytris=.Fot of silex; A/^of, x^i':- 



