structure of the Diatom Shell, i&e. By Dr. J. D. Cox. 403 



sive. These natural sections also often exhibit the horn or process 

 at the corner of the valve in a very interesting way. 



With this paper 1 send to the Society photographs in continua- 

 tion of the series formerly sent, and which seem to me to illustrate, 

 and fairly to confirm, the observations which I have detailed above. 

 I will only notice one other point in diatom structure which is 

 shown in a noteworthy manner. In two of the prints of Pleuro- 

 sigma formosum W. Sm,, will be found a specimen in which a 

 thread of silex forming part of the mechanism of the raphe or 

 median line is for a considerable distance separated from the half- 

 valve to which it belongs. At the point where it is still connected 

 and where the other half of the broken shell is also in place, the 

 lapping of this second half of the valve over the rod or thread may 

 be traced.* The bright line which is continuous with the line of 

 separation of the rod from the valve indicates the thinness of the 

 connecting film, which is again proved by the fact of the fracture 

 along it. The whole connection of the parts would thus seem to be 

 plainly shown to be a " tongue and groove " joint from the central 

 nodule to the terminal one. 



For convenience of reference to the photographs by the 

 Fellows of the Society, I give a list of the additions to the series 

 which I send with this paper. Like the former ones, all have been 

 made with the common lamp, and all but Nos. 69 and 71 with a 

 small beam of strictly central light. The two excepted were made 

 with a stop upon the condenser, having two round holes equidistant 

 in the horizontal diameter of the opening of the condenser, such as 

 was recommended for use with the binocular by Dr. E. H. Ward 

 many years ago,t and which is now furnished with Abbe's binocular 

 eye-piece by Zeiss. Using this on the monocular instrument gave 

 in those cases somewhat oblique cross-lights. Whenever it promised 

 good results, the plates were intensified with bichloride of mercury. 



No. 53. Navicula serians Kiitz. x 1565, exposure 16 minutes. 

 A fragment showing fracture through the dots, and the appearance 

 in H. L. Smith's dense medium (2-4). From H. L. Smith's slide 

 of fossil diatoms from Willington, Conn. 



No. 57. N. serians Kiitz. x 1250, exposure 10 minutes. 

 Whole shell with a segment separated by a crack running through 

 the dots. From MoUer's slide of fossil diatoms from Monmouth, 

 Maine, in balsam. 



No. 60. Pleurosigma formosum W. Sm. x 1135, exposure 

 10 minutes. Shell broken across, showing separated median line, 

 and fracture through the dots. From M oiler's slide of Samoa sea- 

 mud, in balsam. 



* Photographs Nos. 60, 79. 

 t Amer. Naturalist, 1870. 



2 D 2 



