328 THE VOYAGE OF H.M.S. CHALLENGED. 



I had given the title Tretodiotyum cyathus. I do not hesitate, therefore, to abandon my 

 generic and specific designation for this form, and to adopt Carter's title, which has the 

 priority. Thus the generic title Tretodictyum is wholly given up, and the two other 

 species which belong to the same genus are no longer called Tretodictyum tuhidosum and 

 Tretodictyum latum, but Hexactinella tuhulosa and Hexactinella lata. 



The specimen figured by Carter, unfortunately not in its entirety, was obtained from 

 Misaki, Japan, at the extrance of the Bay of Tokio. According to his description, it 

 closely resembles the Phakellia ventildbrum, Bwk., figured by Bowerbank in his Mono- 

 graph of British Sponges.^ " The surface is on both sides even and uniform ; uniformly 

 scattered over internally with circular apertures about -^ inch in diameter, and about the 

 same distance apart, and externall)' with a dermal, quadrilateral, spicular reticulation." 

 " Wall about \ of an inch thick, composed of two layers, viz., one on each side of 

 an irregular central plane of condensed tissue, each layer consisting of plumose fibre 

 curving upwards and outwards florally from the centi'al plane of condensed tissue, 

 strengthened by transverse fibres in their course." 



Isolated spicules are represented, according to Carter, by the following types : — (1) 

 dermal pentacts ; (2) uncinates, called barbulte by Carter ; (3) thin oxydiacts ; (4) 

 scopulse with two, three, or four slightly divergent teeth ; (5) simple or spinose oxy- 

 hexacts ; (6) oxyhexasters and discohexasters of diff"erent kinds. 



Character of the Genus. — The wall of each of the cup-shaped or tubular specimens 

 is traversed by canals, which are not exclusively disposed at right angles to the bounding 

 surface. 



The dictyonal framework is principally composed of radial, longitudinal, straight 

 or slightly bent fibrous reticulate 2:)lates, about 1"5 mm. in breadth. These are separated 

 from one another by spaces of similar form and breadth, but are at the same time bound 

 together laterally by numerous transverse beams. A more irregularly developed fibrous 

 network with round openings extends in some species over the outer, in others over the 

 inner (gastral) surface of the dictyonal framework, and thus conceals either on the 

 outside or inside the above mentioned longitudinally directed radial plates and their 

 cleft-like interspaces. 



The dermal and gastral skeleton consists of pentacts or hexacts, and numerous scopulae 

 of various forms. In addition to delicate uncinates, numerous oxyhexasters, disco- 

 hexasters, and more rarely small oxyhexacts and discohexacts appear in the parenchyma. 



1. Heasactinella tuhdosa, n. sp. (PL XCIIL). 



Simple or dichotomously branched, sometimes also laterally anastomosing tubes about 

 1 cm. in diameter, with a wall varying from 2 to 3 mm. in thickness, rise from a flat 



' Vol. iii. pi. xxii. 



