950 SUMMARY OF CURRENT RESEARCHES RELATING TO 



distinct lines of punct^e between parallel costee. It is characteristic, 

 too, of the difference between the transverse and longitudinal strise of 

 Siirirella gemma. It is certainly natural to conclude that tbe similar 

 phenomena are due to similar structure. 



In NaviculcB having strongly radiant costro, some, like N. 

 peregrina, show a similar dotted structure between the ribs, and in 

 these cases the lines of separation between the dots are also much 

 finer and less prominent than the cost^e. In another class of Navi- 

 culce, of which N. sculpta Ehr. is an example, the dots, whilst 

 arranged in lines, do not have thickened costse between the rows, 

 but are like separate, sometimes elongated, punct^e in a shell of even 

 thickness. In these, however, as in N. lyra, the line of fracture 

 follows the dots, and the hyaline parts of the valve show the pink 

 colour, so that both lines of proof still combine to show the dots to 

 be the weak and thin places in the valve. A beautiful example of 

 the latter sort is Mastogloia angulata Grun., which is not uncommon 

 in Long Island Sound, and is found along the whole Atlantic coast. 

 The shell is broad ovate, somewhat cuspidate, of smooth, even thick- 

 ness, and the punctse are arranged in oblique rows. With a medium 

 power the effect is that of a delicate cross-hatching, much like that 

 of P. angulatum. With a high power the dots are well separated, 

 and, except as to arrangement, their appearance is similar to those 

 of iV^. sculjpta. As in the disk forms the diminishing size of the areolas 

 brought us gradually very near to the fine lines of Hyalodiscus suhtiUs, 

 so among the Naviculce we make a similar approximation to the delicate 

 marking of the Pleurosigmas, 



The use of the vertical illuminator upon these diatoms is hardly 

 less decisive in support of the conclusions I have drawn than in the 

 ca^e of the Coscinodisci. A smooth surface, dotted with tiny bubbles, 

 is the characteristic appearance of the shell, and these bubbles, 

 in the larger kinds, cannot be distinguished from those which we 

 have found in the disk forms, beginning with examples from the 

 Nottingham earth, where the hexagons and round areolfe were found 

 side by side upon the same valve. We may even take a step in 

 advance. In Peticolas's slides of Eichmond and Petersburg earths 

 there are numerous examples of a coarse form of P. angulatum var. 

 virginicum, in which the marking in the middle of the valve is coarser 

 than at the extremities. In dry specimens of this shell a high power 

 used with the vertical illuminator will separate the dots sufficiently 

 to show a surface hardly to be distinguished from that of Mastogloia 

 angulata, which I have noticed above. It is a smooth film in which 

 the minute bubble-like dots have the same character, and differ only 

 in size from those in Actinocyclus, "or in the coarser smooth Naoiculce. 

 In some broken specimens, also, the line of fracture could be traced 

 through the dots. 



In Stauroneis jpulcJiella the areolae are much longer in proportion 

 to their width, and are contracted at the ends so as to take the 

 ' oat-shaped ' appearance by which they are commonly known. 

 There is here found a difference in the appearance of the concave 

 and convex sides of the valve, the former presenting the areolae more 



