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



Leuconia rudifera, n. sp. (PL VII. figs. 3a -3c). 



There are in the Challenger collection only some fragments of this interesting species ; 

 they seem to belong to two specimens: in one case the walls are 5 mm. thick, in the 

 other not above 3 mm. One fragment bears an osculum fringed with fine linear acerate 

 spicules. Both the surfaces are rough. With regard to its spiculation, the species is 

 intimately allied to Leuconia typica, but is distinguished from it, as well as from all 

 other Leuconidse, by its remarkable minute verticillate acerate spicules (PL VII. fig. 3 W ). 



Skeleton. — The skeleton consists of gastric verticillate acerate, of gastric quadri- 

 radiate, of parenchymal triradiate, of dermal triradiate, and of acerate spicules of three 

 different kinds. 



Gastric quadriradiate spicules (occasionally to be found also in the walls of the larger exhalent 

 canals). — All rays of the same diameter (0 - 015 mm.); basal ray usually undulating, 

 rarely longer than 02 mm., like lateral rays sometimes sharply, sometimes bluntly pointed ; 

 lateral rays either straight or slightly curved, each forming with basal ray an angle 

 varying from 100° to 105°, length not exceeding (K3G mm.; apical ray curved, tapering 

 from the base to a sharp point, not longer than 01 ram., not seldom rather thinner than 

 facial rays. 



Verticillate acerate spicules. — I call these spicules acerate, for the transitional stages 

 between them and the common spindle-shaped acerate form can be easily found ; these 

 intermediate stages are to be seen on PI. VII. figs. 3a-3a", and it is evident that the 

 three larger teeth on the free end of these spicules are homologous with the smaller teeth 

 on their middle part. Cylindrical in their free half, which projects from the inner 

 surface, these acerate spicules seem to be flat in their more extended half, situated in 

 the parenchyma ; they reach - 064 mm. in length, and O000S mm. and 0'0014 mm. in 

 diameter. 



Triradiate spicules of the parenchyma. — Very inconstant in their outlines, either regular, or 

 sagittal, or irregular. The typical fundamental form can be characterised as follows : — 

 regular ; rays tapering from the base to sharp points, 035 mm. in length, with a 

 diameter of O034 mm. Such triradiate spicules are really to be found, but usually they 

 undergo more or less considerable modifications, either in the direction of a sagittal 

 differentiation, the basal ray growing rather longer than the lateral, and its angles varying 

 from 120° to 110°, or in the direction of an irregular differentiation, all the rays or all 

 the angles growing more or less unequal. In most cases the rays are sharp-pointed. 



Dermal triradiate spicules. — All rays of the same diameter, the proportion between their length 

 and thickness varying from 17:1 to 23 : 1 ; basal ray straight, tapering from the 

 base to an approximately sharp point, forming with each lateral ray an angle of about 

 115°, length not exceeding 0-35 mm.; lateral rays curved, often undulating, usually of 

 the length of basal ray, sometimes rather shorter or longer. In two cases I could discern 

 in the dermal triradiate spicules an incipient apical ray. 



