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



cuneate hoops, which are seen in some species, and the spiral line, which is still more 

 frequent, result simply from the form of the distinct parts, which by their union constitute 

 the tube of the Diatom. That these frustules are composed of several parts that are more 

 or less liable to be detached from one another — a phenomenon which may also be witnessed 

 in the case of the numerous hoops of Rhabdonema, Striatella, and such like forms — can 

 hardly be doubted. The parts that form the tubular walls, instead of being like those of 

 the hoops, are generally somewhat rhomboidal or lozenge-shap3d, and in soma species, e.g., 

 Rhizosolenia robusta, Norman, these, having two sides of the parallelogram extremely long 

 and united by means of two shorter sides, form a belt which is terminated by two 

 transverse lines. In other species every constituent part is exactly rhomboidal and 

 equilateral, while they are bent round and united together in such a manner that they form 

 a tube upon which the line? of suture appear to be arranged in a spiral manner. Very 

 frequently the sides of each rhombus are somewhat curvilinear, and the obtuse angles 

 truncated, yet they fit together in such a manner as to constitute a tube. 



Among the Rhizosolenia, which were found frequently to abound in many surface 

 gatherings made by the Challenger, frustules were often observed in a broken condition on 

 account of their size and the relative tenuity of their walls, aud in such cases the 

 rhomboidal parts of which they were composed were frequently isolated from one another, 

 while at the same time the sutural lines could be disfciuo-uished. It must also be recorded 

 that in some species the rhomboidal plates were found to be very small, so that the 

 perimeter of the tube was formed of several plates, and in such cases (see Plate XX.X. figs. 

 11 and 14) the surface of the Diatom resembled the skin of a fish or of a serpent. 



The genus Rhizosolenia was always regarded as marine until Professor Hamilton 

 Laurence Smith discovered his very singular Rhizosolenia eriensis 1 and subsequently 

 Rhizosolenia gracilis, which more rarely accompanied the former, in a surface gathering 

 made on Lake Erie in North America. 



In marine gatherings the genus, in addition to its occurrence as a surface form, has 

 also been recognised amongst the contents of the stomach and alimentary canal of 

 Mollusca and other marine animals. In soundings, and still more in deposits, the Rhizo- 

 solenice can only be recognised by means of their mucrones, which, like a solid or massive 

 substance, can resist the trituration and pressure that invariably reduce their thin walls 

 to very minute and unrecognisable debris. 



Another feature of great importance, and one which has not hitherto been recognised, 

 is well seen in some of the species that have now to be recorded, namely, the existence of 

 a minute cavity towards the extremity of each frustule. This cavity — if it is not to be 

 regarded as a consequence of the union — no doubt serves for the more perfect adhesion of 

 the frustules, which are disposed in rows or series, as each corresponds in position to the 

 extremity of the terminal region of an adjoining frustule. 



, 1 See figure in van Heurck's Synopsis des DiatonieWde Belgique, pi. lxxix. fig. 9. 



