952 SUMMARY OF CURRENT RESEARCHES RELATING TO 



index than balsam this becomes still more noticeable. In the 

 Pleurosigmas I have named, I think a similar thickening of the lines 

 (much more delicate, but real) has taken place, and that this gives 

 the strong cross-hatching which marks them. In the varieties 

 more closely allied to P. angulatum the shell is smoother, and in 

 some of these the surface, with high magnification, and both by trans- 

 mitted light and under the vertical illuminator, is found to resemble 

 very closely that of the distinctly areolated forms which have been 

 described. 



In conclusion, I will notice briefly a few of the less regularly 

 marked diatoms, but which still seem to me to corroborate the view of 

 their structure which I have maintained. 



In a group of species allied to Navicula prcetexta Ehr., including 

 N. Kennedyi, N. indica, N. clavata, &c., the regular striae are confined 

 to narrow bands at the margin and along the median line, the inter- 

 mediate space being either hyaline or mottled in varying degrees of 

 distinctness. Specimens which have this mottling most distinct 

 exhibit it as a system of rather large but faint dots, arranged in lines 

 continuous with the distinct striae at the margin, &c., but the dots in 

 these lines are irregularly spaced as to distance. Occasionally an 

 individual is found in which the dots are as sharply defined as in any 

 of the smooth Naviculce, and giving the proof that they are areolae by 

 fracture and by colour. Arranged in a series, therefore, they show 

 us that the diminishing distinctness of marking is due to the pro- 

 gressive shallowness of the depressions in one of the laminae of the 

 valve, until from faintest mottling the dots disappear entirely, leaving 

 the interior space smooth and hyaline. 



The study of these last assists us in understanding the marking of 

 Eeliopelta. In this splendid shell we have, first, an outer lamina or 

 film, finely punctate, making the appearance of diagonal cross- 

 hatching upon each of the undulating segments. This film is some- 

 times found partially separated from the under one, much as the 

 laminae of Coscinodiscus are found. In the Nottingham and Calvert 

 County earths I have found this separation extending over part of a 

 segment of the shell, a whole segment, two or three segments, and in 

 one instance the whole valve. In this last case the separate outer 

 lamina is not distinguishable from the figure given as Actinoptyclms 

 pellucidus Grun., by Van Heurck, and I cannot doubt that this latter 

 is a separated plate of a similar valve. The separation has included 

 the central hyaline star figure in the shell as well as the dotted 

 part, showing that the laminae exist here also, notwithstanding the 

 homogeneous transj)arence of this part of the valve. In the second 

 place, the inner lamina is found to have a different marking in the 

 undulating segments. Those projecting outwardly from the face of 

 the frustule are areolated with a sub-hexagonal areolation, quite 

 distinctly defined. Those which are depressed have usually a much 

 shallower sculpture, of which the normal marking is an hexagonal 

 arrangement of large shallow dots, but these are sometimes enlarged 

 into a system of more distinctly marked equilateral triangles com- 

 bined, so that the six form a regular hexagon. The difference between 



