REPORT ON THE HEXACTINELLIDA. 189 



Hexactinellid, which he names Hyalonema arcticum, belongs to the genus Hyalonema. 

 No Hyalonema possesses a " round hollow stem," with rough, rod-like spicules, rounded 

 off at both ends ; and in no Hyalonema are there rough discohexacts, such as are 

 figured {loc. cit., fig. 10a). On the other hand, I conjecture from the description and 

 from the figures of Armauer Hansen that we have to deal with a species of my genus 

 Caidophacus, and certainly at least with an Asconematid. 



Character' of the Genus. — The body is generally in the form of a thick-walled cup, 

 from the shallow cavity of which a central conical boss projects. From the upper 

 sharply defined border, which is distinguished by a fringe of marginal spicules, an 

 independent sieve-net-like perforated membrane extends in many species over the 

 whole oscular opening. From the centre of the lower, usually somewhat narrowed end 

 of the body, a tuft of long and strongly developed basal spicules projects downwards. 

 The tuft consists wholly or for the most part of four-toothed anchors, which serve to 

 moor the sponge in the loose mud. The bundle of spicules is continued superiorly into 

 the cone which rises in the gastral cavity. In most species, and perhaps even in all, the 

 upper portion of this basal tuft is surrounded by an encrustation of Palythoa, which 

 begins just below the end of the body proper, extending in difierent individuals to a 

 very varied distance, but never growing on the lower somewhat bushy divergent portion 

 of the tuft. 



Both dermal and gastral skeleton are composed (l) of strong pentacts, with the 

 unpaired ray sunk more or less deeply into the parenchyma, (2) of pinuli which have 

 their tangential basal cross inserted in the .skin, and (.3) of amphidiscs which are 

 radially disposed with the median axial cross portion embedded in the skin, and with 

 one end projecting freely outwards, while the other extends into the parenchyma, or into 

 the subdermal or subgastral spaces as the case may be. Amphidiscs more rarely occur 

 tangentially in the skin, but are then represented only by minute forms. In some species 

 the gastral skeleton is continued without much change into the eff"erent ducts, i.e., into the 

 canalicular skeleton. The parenchyma contains large and small oxyhexacts, cruciate 

 tetracts, triacts, straight diacts, and occasionally isolated monacts. 



The chambers are not so distinctly marked ofi" as glove-finger or thimble-shaped 

 sacs as in most of the other Hexactinellids. They appear rather as more irregular 

 and less sharply defined diverticula of the membrana reticularis. 



I shall divide the genus Hyalonema, Gray, into the two subgenera Hyalonema, sens, 

 strict., and Stylocalyx — a distinction based on the presence or absence of an oscular 

 sieve-plate. 



