108 



THE AMEKICAN MONTHLY 



[June, 



punctate, making the appearance of 

 diagonal cross-hatching upon each of 

 the undulating segments. This film 

 is sometimes found partially separated 

 from the under one, much as the 1am- 

 inte of Coscinodiscus are found. In 

 the Nottingham and Calvert County 

 earths I have found this separation ex- 

 tending 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 distin- 

 guishable from the figure given as 

 Actinoptychus pellucidus, Grun., by 

 Van Heurck, and I cannot doubt that 

 this latter is a separated plate of a sim- 

 ilar valve. The separation has in- 

 cluded the central hyaline star figure 

 in the shell as well as the dotted part, 

 showing that the laminae exist here 

 also, notwithstanding the homoge- 

 neous transparence of this part of the 

 valve. In the second place, the inner 

 lamina is found to have a difierent 

 marking in the undulating segments. 

 Those projecting outwardly from the 

 face of the frustule are areolated with 

 a sub-hexagonal areolation, quite dis- 

 tinctly defined. Those which are de- 

 pressed have usually a much shallow- 

 er sculpture of which the normal 

 marking is a hexagonal arrangement 

 of large shallow dots, but these are 

 sometimes enlarged into a system of 

 more distinctly marked equilateral 

 triangles combined, so that the six 

 form a regular hexagon. The differ- 

 ence between the deeper and shallow- 

 er areolae in this case is similar to that 

 which has been described in Navic- 

 ula pi'cetexta, etc. , and when they are 

 covered by the lace-like veil of the fine- 

 ly dotted film we have the beautiful and 

 changeable effect which has proven so 

 puzzlingto observers. In whole valves 

 of Heliopelta the lai'ger areolae will 

 often be found showing in the central 

 part of the shell where the fine dotting 

 of the upper film does not extend over 

 them, and their character may there 

 be pretty satisfactorily determined, 

 even if the separated laminae are not 

 detected. 



Since this article was in type I have 

 received from Mr. Thomas Christian, 

 of Richmond, Va., a slide containing 

 a valve of Triceratium favus ^ which, 

 whilst he was endeavoring to pick it 

 up, split into two films, the inner 

 with its marking of dots in radial 

 lines wholly separating from the 

 outer, which has the deep hexagonal 

 cells closed with the exterior film 

 with marking of eye-spots. The in- 

 ner film has also the outline of hexa- 

 gons upon it, being the mark of the 

 attachment to the hexagonal cells. 

 In this it differs from Heliopelta^ in 

 which the dotted film shows no mark 

 of connection with the hexagonal are- 

 olae below. Mr. Christian's speci- 

 men of Triceratium is the first ex- 

 ample of the entire separation of the 

 laminse which I have met in that spe- 

 cies. 



In Halionyx a similar structure is 

 found, but in the alternate segments 

 the irregularity of sculpture of the 

 inner lamina of the shell is carried 

 further. It now has no geometric 

 arrangement of areolae, but these are 

 shallow and of irregular shape, yet so 

 adapted to each other as to produce 

 a general harmony of effect that is 

 very beautiful, especially when seen 

 through the veil of the regular and 

 finely-dotted film above. In some 

 varieties a strong hyaline radial line 

 extends more than two-thirds the way 

 from centre to margin in the alternate 

 segments, and that this is stronger 

 than the dotted part is proven by its 

 being often found, in broken shells, 

 projecting boldly beyond the rest of 

 the fractured film. In similar exam- 

 ples the fracture, through even the 

 finer dots, is demonstrably plain, and 

 this last as well as that of color de- 

 termines the agreement of both He- 

 liopelta and Halionyx with the gen- 

 eral law of structure which I have 

 attempted to develop. The other 

 species of Actinoptychus are strictly 

 analogous to the two described. 



The effect of shallow sculpture of 

 the shell is seen also in many of the 

 melosireae when radial lines and dots 



