REPORT ON THE DIATOMACE.E.j 73 



portion surmounted by short sharp points. Although the extremities can hardly be called 

 calyptriform, the generic determiuation that has here been given cannot be questioned, 

 more especially as an abundant supply of siliceous matter has been proved to occur in its 

 walls by subjecting it to a process of incineration. 



Rhizosolenia imbricata, Bright. (Plate XXIV. figs. 1 and 1 bis.) 



The form which is rej)resented on Plate XXIV. fig. 1 under a magnifying power of 

 130 diameters, and at fig. 1 bis under a power of 640 diameters, belongs without doubt 

 to the Rhizosolenia imbricata of Brightwell. 1 In connection with fig. 1 the unimportance 

 of attaching any significance to the proportion between the transverse axis and the length 

 of the frustule may be noted, and it may also be observed that in the middle of the 

 granulated series of rings there occurs a smooth ring. Here it is probable that the 

 process of division takes place, by the formation of a diagonally placed diaphragm, which 

 arises in such a manner as to bring about the formation of two new calyptrse. 



The arrangement of the moniliform striae on the other rinses is radiating;, the radiation 

 taking place outwardly from a median axial line. Each ring is flanked by two small 

 triangular and similarly sculptured spaces, from which it may be inferred that the entire 

 annulus is composed of two trapezoidal parts, on each of which the striae run in different 

 directions. 



Plate XXIV. fig. 1 4, represents a form bearing on its calyptriform extremity two long 

 and very acute mucrones. Whether this appearance is normal or teratological, or whether 

 it is merely accidental— one mucro alone belonging to the frustule, the other being 

 casually apposed — cannot be determined. Should further observations prove that both 

 belong to the same Diatom, it must be regarded as a new species, but at present there is 

 nothing to show that it is not a monstrous form. 



o 



Rhizosolenia robusta, Norman, var. nov. (Plate XXIV. fig. 5.) 



Here there is shown the calyptriform extremity of a Rhizosolenia, which is remarkable 

 on account of its extraordinary size and the sculpturing of simple radial lines which it 

 presents. These characters coincide with those of Rhizosolenia robusta, Norman, which 

 is figured in Plate viii. fio;. 42 of Pritchard's Historv of the Infusoria. The form of the 

 latter, however, unlike that of the former, is compressed, and its mucro is described as 

 being " short, delicate, and nearly linear," instead of being short and cuneato-acute, as in 

 the present case. Yet these points of difference cannot be regarded as possessing more 

 than a varietal significance. 



The parts entering into the composition of the rings of this variety are sub- 

 rectangular. 



1 Micr. Joum., vol. vi. p. 94, pi. v. fig. 6. 

 (bot. chall. exp. — pabt iv. — 188G.) D 10 



