32 W. M. BALE ON SOME OF THE DISCOID DIATOMS. 



Jl, fasciculatus Castracane is distinguished by the notable width 

 of these areas, but the character is of no specific importance. 

 A, Ehrenbergii often exhibits such areas, and I have seen them 

 in one valve while the other in the same frustule showed 

 scarcely a trace of them. They may even exist on only a part 

 of a valve. So far as Castracane's figures show, there is nothing 

 to distinguish his species from A. Ehrenbergii. 



A frequent phenomenon in the genus is the occurrence of 

 regular or irregular blank areas crossing the rows of puncta, 

 often in a sub-concentric fashion, and A. crassus is a form in 

 which the apparent irregularity of the markings from this cause 

 has been made a ground for specific distinction. Yet both Van 

 Heurck and Peragallo, who admit the species, show by their 

 figures that the markings are as in A . Ehrenbergii, except in so 

 far as the granules ard obliterated over certain irregularly sub- 

 concentric areas. I find nothing here to warrant the separation 

 of the form as a distinct species. 



The interfasciculate rays are also liable to interruptions, and 

 Castracane has described a species A. complanatus in which 

 they are said to be wanting, though the valve is of the ordinary 

 fasciculate type. I greatly doubt the correctness of this, not 

 merely on a priori grounds, but owing to Rattray's identification 

 of this species with the form distributed by Moller as A. Ralfsii. 

 Now the " A. Ralfsii 1 ' of my Typen-Platte is simply one of the 

 forms of A. Ehrenbergii in which the fasciculation is similar to 

 that of A. Ralfsii, and which abound in Cuxhaven and Ichaboe 

 guano material. The interfasciculate rays are certainly not 

 wanting, though doubtless obscure and irregular in parts. Many 

 otherwise similar valves occur in which there is no noticeable 

 irregularity of these rays. 



The general aspect of the valve depends largely on the position 

 and distance of the granules relatively to the others in the same 

 und adjacent rows of the fascicle. In A. Barklyi the granules of 

 each row are very close to each other, but not so close to those 

 of the next rows ; the rows therefore remain distinct from each 

 other even to the border. In A. Ralfsii type and var. sparsus 

 the granules of adjacent rows are mostly side by side, so that 

 they form straight lines crossing the fascicles, thus having as a 

 whole a sub-concentric disposition ; they are also distinctly 

 separated from each other. In A. Ehrenbergii there is much 



