EEPOET ON THE CALCAEKA. 07 



Gastric quadriradiate spicules. — Regular, facial rays straight, smooth, tapering from the base to 

 approximately sharp) points, their average length being 0T5 mm , ami their diameter 

 0'01-0'013 mm.; apical ray of the same diameter, sharp-pointed, either straight or curved, 

 often irregularly bent ; sometimes very short, rudimentary, usually 0-2-0-25 mm. long. 



Triradiate spicules of the pitraicliyma. — The greater part of these consists of spicules not 

 differing from the quadriradiate ones just described, except in the absence of the fourth 

 ray; amongst them are scattered here and there much larger triradiate spicules, which 

 differ from them only in size. The rays of these larger triradiate forms sometimes 

 exceed 1 mm. in length, the proportion between the length and thickness varying from 10:1 

 to 12 : 1 ; they are connected with the smaller triradiate variety by intermediate stages. 



Dermal and subdermed triradiaU spicules. — lu the variety hoinoraphis these do not differ, either 

 in their size or iu their form, from the smaller triradiate ones of the parenchyma ; in the 

 variety heteroraphis they remain of the same size, but become sagittal and irregular. 

 Their typical modifications are represented on PI. VII. fig. 8. They form iu this variety 

 also the skeleton of the collar; the fragment of the var. homoraphis I possess has its 

 oscular part broken off. 



Colour. — Greyish. 



Habitat. — Station 135, October 1873; Island of Tristan da Cunha ; depth, GO 

 fathoms ; rock, shells. 



Leucetta, Haeckel. 



Leuconidse, the skeleton of whose strongly developed cortex is quite different from 

 that of the parenchyma. 



Leucetta imperfecta, n. sp. (PL VII. tigs. 9a-9e). 



This species, like the following, Leucetta vera, is represented in the Challenger 

 collection by a single specimen, which is bare-mouthed, of tubular, elongated, cylindrical 

 form, 35 mm. long and 5 mm. in diameter, the thickness of the wall being T25 mm., 

 that of the cortex 0'35 mm. Both the surfaces are rather rough. The characteristic 

 peculiarities of the species consist in the form of its pigmy triradiate, and in the presence 

 in the parenchyma of large quadriradiate spicules, not differing either in size or in form 

 from those of the cortex; these last are not numerous, but they are there, and most 

 of them having just the same disposition as the large subgastric quadriradiate spicules in 

 Leucilla amphora, H. (Kalkschwiimmc. iii. pi. xxiv. fig. 8), the species may be regarded 

 as a connecting link between the genera Leucetta and Leucilla. 



Skeleton. — The skeleton consists of minute gastric quadriradiate, of minute triradiate 

 and quadriradiate and of large quadriradiate spicules of the parenchyma] of cortical 

 triradiate spicules lying in several layers parallel to the outer surface, and of cortical quadri- 

 radiate spicules, their facial rays lying in the cortex parallel to the cortical triradiate 



