1210 THE VOYAGE OF H.M.S. CHALLENGEE. 



The genus Spongomelissa differs from its ancestral genus Lithomelissa only in the 

 development of spongy framework in the shell-wall — a very rare production in the 

 Cy rtoidea (compare Peripyramis, p. 1162, and Spongocyrtis, p. 1188). 



1. 



Spongomelissa spongiosa, Haeckel. 



Lithomelissa spongiosa, Biitsclili, 1882, Zeitschr. f. wiss. Zool., vol. xxxvi. pp. 519, 539, Taf. 

 xxxiii. fig. 25, a, b, c. 



Shell of dense spongy structure, with a deep collar stricture. The subspherical cephalis and the 

 truncate abdomen of nearly equal size, both with irregular delicate spongy framework. Mouth 

 wide open. Cephalis with a large vertical apical and a small oblique frontal horn. Thorax with 

 three very stout, three-sided prismatic widely divergent lateral wings, which are covered with 

 numerous irregularly ramified branches ; the spongy framework arises hy communication of the 

 dehcate branches. 



Dimensions. — Cephalis O'Ol: long, O'Oo broad, thorax 0'04 long, 0'06 broad. 



Habitat. — Fossil in Barbados. 



Genus 540. Clathrocanium,^ Ehrenberg, 1860, Monatsber. d. k. preuss. 

 Akad. d. Wiss. Berlin, p. 829. 



Definition. — S e t h o p i 1 i d a (vel Dicyrtida triradiata aperta) with three prominent 

 lateral ribs on the thorax, alternating with three large holes (or thoracic gates). Cephalis 

 with a horn. 



The genus Clathrocanium is nearly allied to Dictyophimus and Lithomelissa, and, 

 together with these two genera, may be regarded as surviving representatives of the 

 oldest and most primitive forms of Dicyrtida. It differs from the latter mainly in the 

 incomplete fenestration of the sheU, three large interradial holes remaining between the 

 three perradial thoracic ribs. It may therefore be derived either from Eusceniuni or 

 from Tripospyris by the development of a terminal lattice-band between the three feet. 

 Clathrocanium may be divided into two different subgenera : Clathrocanidium, with 

 simple horn and smooth mouth, and Clathrocorona, with fenestrated horn and coronated 

 mouth. 



Subgenus 1. Clathrocanidium, Haeckel. 

 Definition — Horn of the cephalis simple, not fenestrated. Peristome smooth. 



1 Ctot/irocciiwMm = Basket of network ; xXij^^a, x.»vtoii. 



