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



on which the inward extension of the border must never encroach. 

 I have had no opportunity of proving whether this is so, or even 

 o: ascertaining whether the apiculi of the two valves are opposite, 

 except in a single instance a large cylindrical frustule of C. 

 mirificus mounted in zonal view, and in this the apiculi are 

 opposite. 



In C. gigas the apiculi are, if present, obscure, and I can find 

 no marginal indications of them. C. diorama and allied forms, 

 however, often classed as varieties of C. gigas, have the border 

 distinctly marked with two apparent notches as in C. perforatus. 

 C concinnus has distinct apiculi, and many specimens have in 

 addition crescentic processes outside the valve, partly surrounding 

 the point at which the apiculi originate. These valves are known 

 as Eupodiscus Jonesianus Greville (E. commutatus Grunow), but 

 I do not think they have any claim to rank even as a variety. 

 They are abundant in slides from Ouxhaven, mixed indis- 

 criminately with valves having the internal apiculi only. 



While several forms besides those which I can identify with 

 the foregoing species share in the peculiarity in question, there 

 are many others in which I have failed to detect it. Such are the 

 thick variety of C. Oculus Iridis found in the Mors deposit, also 

 C. radiatus. In more typical forms of C . Oculus Iridis, however, 

 careful search has disclosed two apiculi, which are simple bacillar 

 projections into the cavity of the frustule. 



Apart from these appendages the structure of the border itself 

 has in many cases not received sufficient attention as a help in 

 classification. Some species have distinct borders with markings 

 quite different from those of the valve generally, others have the 

 areolar structure continued to the extreme margin without 

 interruption ; in some the edge is turned over, in others it is 

 quite flat, and frequently the specific diagnosis contains no 

 hint of the character of the valve in this respect ; so that of two 

 valves, differing widely in this particular, it may be impossible to 

 decide which of them corresponds with the specific description. 

 C. concinnus and C. centralis may serve to illustrate this. Both 

 are very convex, but in the former the marginal part is slightly 

 flattened, the areolae diminish to a very minute size, and are 

 succeeded by an extremely narrow hyaline border, thinning 

 away so as to show only a smooth single contour. In a typical 

 C. centralis, on the other hand, the valve curves downward to the 



