432 E. M. NELSON ON MINUTE DIATOM STRUCTURE. 



the true images. For example, in the ghost images of intercostals 

 a missing one is never found, but in the true image missing ones 

 and other irregularities are common. 



Eupodiscus Argus. — Here we find a triple structure — viz., 

 primary, secondary, and tertiary markings.* The primary is 

 large enough, and presents no difficulty ; the secondary can also 

 be very easily seen, but its true nature has, I think, never been 

 thoroughly cleared up ; the tertiary is the ordinary finely 

 perforated membrane, but this is hardly so fine as usual. In 

 this diatom we have two, and only two membranes — viz., that 

 on the outside of the valve : which contains the primary and large 

 areolations, and that on the inside, which has the secondary and 

 tertiary structure. The secondary markings are tubes which 

 pass obliquely through this membrane and open into the large 

 areolations. They diverge towards the inside of the valve, and 

 consequently are converging towards the outside. 



We come now to a very difficult example — viz., the Auliscus 

 sculptus. I have at last succeeded in resolving the rose pattern 

 in the processes of this diatom. Having been able to demonstrate 

 the similar, but much coarser, structure in the A. racemosus^ 

 it was a moral certainty that other varieties had it also. In the 

 Sculptus, however, it is so exceedingly fine that it has hitherto 

 baffled all attempts at resolution ; but now it has been seen on 

 two or three occasions. 



The A. sculptus has also very fine perforations in its beautifully 

 sculptured border. Another variety — viz., the A. macrceanus — is 

 a good one to begin upon, because the rose patterns in the processes 

 are coarser than those in A. scutytus. All the above diatoms 

 were mounted in balsam. It may be of interest to note that a 

 bar has been met with crossing an areolation in the P. angulatum 

 precisely similar to that in P. formosum, recorded in 1886.| 



* Journ. Q.M.C., Ser. II., vol. 2, p. 270, fig. 4, pi. 17 (1886). 

 f Joum. Q. M. C, Ser. II., vol. 4, p. 31G, fig. 12, pi. 20 (1891). 

 t Journ. Q. M. C, Ser. II., vol.i2, p. 257 (1886). 



