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



Rhizosolenia (?) flaccida, u. sp. (Plate XXIX. fig. 4.) 



Cylindrica, aniiulata, parietibus tenuissimis firrnitate destitutis ; annulis brevibus 

 parallelis. In mari Adriatico et Arafura. 



Among the surface gatherings which have been made at intervals for some years past in 

 the Adriatic, I have observed the frustule here figured from a form found on the surface of 

 the Arafura Sea. It possesses a delicate structure, being bounded by two nearly parallel 

 longitudinal lines, and crossed by transverse parallel lines, which are disposed at approxi- 

 mately equal distances from one another. Towards each extremity there is an irregular 

 circle, whose diameter is to that of the frustule in the ratio of two to three. These circles 

 are the extremities of the tubular organisms, and, like the parietes, are so thin that 

 when dried on a glass they collapse. It may be noted, however, that the parietes, though 

 very delicate, contain siliceous matter, as they resist exposure to a red heat. 



Although the tubuliform part of this interesting organism bears a resemblance to the 

 Rhizosolenice, no calyptriform extremities have ever been observed, hence its true generic 

 position cannot be determined with certainty. It can only be examined when mounted in 

 the dry state. 



An organism which is pretty abundant in the surface of the Sea of Arafura is shown 

 in Plate XXIX. fig. 8. It is of ovoid form, and terminates at one of its slightly prolonged 

 extremities in a small flat circular area, from which a strong solid conical point arises. At 

 its opposite extremity an opening is seen, which doubtless serves for the reception of the 

 salient extremity of the succeeding frustule. The parietes are slightly and indistinctly 

 striated, but the small terminal circle is surrounded by short but somewhat more pro- 

 tuberant lines. Although the appearance presented by the pointed extremity recalls the 

 calyptra of a Rhizosolenia, it is at present impossible to determine whether the organism 

 really belongs to this genus or is a sporangial form. It is clear, however, that it 

 is a normal form, as many similar specimens have been observed. 



Rhizosolenia arafurensis, n. sp. (Plate XXX. fig. 12.) 



Frustula cylindrica, non annulata ; parietes partibus subtrapezoidalibus composita? ; 

 calyptra desinit in elongatum, obtusum, tubularem mucronem ; striae impervise. In mari 

 Arafura. 



Although in all other recent and fossil Rhizosolenia? the terminal mucrones are solid, the 

 form shown at fig. 12 has this part tubular and obtuse. The frustular walls are composed 

 of subtrapezoidal pieces, the separating quasi-sutural lines being merely thinner and more 

 transparent strands. The striae ornamenting the various parts are exceedingly delicate, 

 but their presence is argued by the slight tint that may often be recognised under the 

 microscope. 



On Plate XXX. figs. 11 and 14, two other forms of Rhizosolenia are represented, in 



