REPORT ON THE EADIOLARIA. 1233 



its upper half with three divergent ribs in the wall, half as broad as the subspherical lower half. 

 (The three ribs probably were overlooked by Ehrenberg.) 



Dimensions. — Cephalis 0"03 long, OOo broad ; thorax Ol long, 0-07 broad. 



Sahitat. — North Pacific, Station 256, surface ; Californian Sea, 2600 fathoms, Ehrenberg. 



4. Sethopera lagena, Haeckel. 



Lithopera lagetia, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 78, Taf. iii. fig. 4. 



Cephalis pear-shaped, with a small conical horn of half the length. Length of the two 

 jioints = 3:8, breadth = 3:6. Thorax smooth, pear-shaped, with irregular roundish pores ; in the 

 wall of its upper third three divergent thin ribs (sometimes wanting) ; the lower half inflated. 

 (Compare Scthocapsa lagciui.) 



Bimensions. — Cephalis 0'03 long, 0'03 broad ; thorax 0"08 long, 0"06 broad. 



Habitat. — Fossil in Barbados. 



Genus 551. Lithoperet,^ Ehrenberg, 1847, Monatsber, d. k. preuss. Akad. d.. 



Wiss. Berlin, p. 54. 



Definition. — S ethoperida (vel Dicyrtida triradiata clausa), with three divergent 

 ribs enclosed in the cavity of the thorax. Cephalis wdth a horn. 



The genus Lithopera is, like the preceding Sethopera, one of the most simple and 

 oldest forms of the Sethoperida, and exhibits three divergent thoracic ribs, which are 

 prolonged neither into lateral wings nor into basal feet. But whilst in Setho2iera the 

 three ribs are enclosed in the lattice-work of the shell-wall, in Lithopera they lie freely 

 in its cavity, and are overgrown by the enveloping shell-wall, being inserted on its inner 

 face with their distal ends. Lithopera may be derived directly from Lithomelissa, by 

 development of lattice-work closing the shell-mouth. Therefore the lower part of the 

 cephalis is hidden in the uppermost part of the thorax ; only its upper part is free. 



1. Lithopera bacca, Ehrenberg. 



Lithopera bacca, Ehrenberg, 1872, Abhandl. d. k. Akad. d. Wiss. Berhn, p. 297, Taf. viii. fig. 1. 



CephaKs spherical, with small, irregular pores and a bristle-shaped spine of half the length ; its 

 lower half hidden in the thorax. Length of the two joints = 1:6, breadth = 1:5. Thorax 

 ellipsoidal, smooth, with regular, hexagonal, small pores, and very thin, thread-like bars. From the 

 deep collar stricture there arise three internal, divergent bars, which are inserted at the uppermost 

 third of the thorax. 



Dimensions. — Cephalis 0'02 diameter ; tliorax 012 long, O'l broadi 



Habitat. — Tropical Pacific, Stations 200, 224, 266, 271, &c., surface.. 



1 ii(/iD;pera = Stone-pouch ; x/tfoj, ■nijoa.. 



(ZOOL. CHALL. EXP.- — PART XL. — 1886.) Rf 155 



