REPORT ON THE HEXACTmELLIDA. 107 



Genus 4. Rhahdodictyum, 0. Schmidt. 

 This genus contains only one species. 



Rhahdodictyum delicatum, 0. Schmidt (PI. XX.). 



Of the two Hexactinellids (dredged in the neighbourhood of Bequia from a depth 

 of 1591 fathoms) which Oscar Schmidt has distinguished as varieties of one species — 

 Rhahdodictyum delicatum — it appears to me, as already mentioned, that the variety A is 

 identical with my Dictyocalyx gracilis. For the variety B I retain the original name. 

 0. Schmidt describes this latter form as a " simple or l:)ranched tube with a perforated 

 wall, formed of interwoven cords. The cords consist of amalgamated or loosely fused 

 hexradiate spicules. The rays, which are arranged in layers in the longitudinal direction 

 of the cords, are for the most part strikingly prolonged, so that the tissue has the appear- 

 ance of being composed of irregularly intersecting rods. The free hexradiate spicules are 

 slender, smooth when quite young, but subsequently covered with spines. The rays are 

 very pliable. A beautiful rosette form occasionally occurs, in which each of the six rays 

 is provided with eight intersecting umbels." 



Oscar Schmidt's figure,^ and still more his description, have convinced me that these 

 skeletons dredged by the Challenger in the neighbourhood of the Bermuda Islands 

 (Station 56), from a depth of 1075 fathoms, and on coral mud, belong to Rhahdo- 

 dictyum delicatum, 0. Schmidt. They exhibit slender, almost tubular cups, with a length 

 of 6 cm., and much broken at the upper ends (PI. XX. fig. l). The basal portion, which 

 is from 6 to 10 mm. in breadth, is attached by means of a terminal expansion to some 

 solid body, and becomes gradually widened upwards to twice this diameter or more. The 

 wall of the tube, which in the larger specimens measures as much as 4 mm. in thickness, 

 is radially perforated by numerous round holes. These are from 2 to 4 mm. in width, 

 and become somewhat broader towards the exterior, at the same time increasing in 

 diameter towards the upper end of the sponge. The arrangement of these parietal 

 apertures is tolerably irregular in the inferior portion, but towards the superior extremity 

 acquires more and more the character of two somewhat steep, intersecting sjjiral rows. 

 The siliceous framework which forms the supporting wall is usually from 1 to 3 mm. 

 broad between these foramina, and consists of the greatly prolonged hexact-rays firmly 

 united by soldering and synapticula (PI. XX. figs. 2, 3. 4). 



Since the three specimens which are at my command are all incomplete and wholly 

 macerated there is no trace of the soft tissue nor of the looser spicules of the parench}'Taa, 

 skin, or gastral membrane, so that no conception of the structure of the entire sponge can 

 be obtained suflicient to enable us to determine whether we have here to deal with a 



' hoc. cit., pi. vii. fig. 3, B. 



