REPORT ON TlIK DIATOMACE.E. 45 



on the valve of Glyphodesmis — its generic determination must remain uncertain. This 

 being so, the specific characters, which can Le better divined than decided with certainty, 

 may at present be left out of consideration. 



Plagiogramma, Grev. 



Greville 1 instituted this genus in 1859, and defined it as follows: — Frustules quad- 

 rangular, direct, two or more united into a filament ; valves linear or elliptical ; striae 

 moniliform ; vittae two or more, pervious, parallel with the striae. 



Plagiogramma thaitiense, n. sp. (Plate XIX. fig. 4.) 



Frustulis rectangularibus subinflatis ; vittis duabus centralibus perviis ; striis trans- 

 versis tenuioribus ad extremum usque. Ad portum Thaiti. 



This Diatom was obtained at the port of Tahiti. It recalls the Plagiogramma 

 jamaicense" of Greville in the fact that the striae, which appear on the margins of the 

 connecting zone, are continued to the angles. In the type now under examination, 

 however, whde the striae extend absolutely as far as the extremities, they are much more 

 delicate than in the Grevillean species, while the margins of the frustule, when viewed in 

 the zonal aspect, are somewhat protuberant in the centre, whereas in Plagiogramma 

 jamaicense they are straight if not even slightly concave. 



Although each of these points of distinction when taken by itself may be of small 

 value, when they are considered collectively they must be regarded as sufficient for the 

 establishing of a new species. 



Plagiogramma margaritaceum, n. sp. (Plate XIX. fig. 13.) 



Valvis late ellipticis, convexiusculis ; granulis distinctioribus quadrato ordine dispositis ; 

 granuli inter medias costas vacant, ad apices evanescunt. In portu Thaiti. 



This frustule, which is also from the port of Tahiti, possesses an elliptical outline, and, 

 as indicated by the curves of the transverse lines which separate the rows of granules, is 

 markedly convex. The striation is continued to the extreme apices but the striae are not 

 moniliform — a fact which, however, cannot be regarded as of essential importance. On 

 this ground Plagiogramma tessellatum, to which Gregory affixed a note of interrogation, 

 must also be admitted to be a true Plagiogramma. 



1 Micr. Journ., vol. vii. p. 20S. Greville here divided the genus into three sections, according to the 

 character of the vittse, viz., Sect. I. Vittse two, centrical; Sect. II. Vittae two, centrical, and one at each end 

 of the valve ; Sect. III. Number of vittae between the two centrical ones and the ends of the valve indefinite. 



2 Micr. Juurn., vol. vii. p. 208, pi. x. fig. 3. 



