150 A. COTTAM ON A NEW AULACODISCUS, 



to the Museum and examined carefully every specimen in Dr. 

 Greville's collection, both of A. Kittoni and A. Johnsonii — about 

 half-a-dozen of each. 



It is the result of these enquiries that I propose to submit to 

 you this evening, in hopes tbat I shall be able to satisfy you that 

 the diatom in question is a new species distinct from either of those 

 to which it has been referred. 



Both species, A. Kittoni and A. Johnsonii, were named by the 

 late Professor Walker- Arnott ; and as Br. Greville's collection 

 contains examples of each mounted by Dr. Arnott, I conclude that 

 I have examined typical specimens. 



The accompanying figures 1 and 2, Plate 12, are a large speci- 

 men, and one of the smallest specimens I could find of the new 

 form. Figures 4 and 6 are varieties of A. Kittoni, the small one 

 being a recent specimen from New Zealand ; the large one a 

 specimen out of the so-called fossil deposit from Mexillones, in 

 Bolivia. 



I will now quote Dr. Arnott's description of A. Kittoni : — " Disc 

 hyaline, with 3 to 8 submarginal crescent-looking processes, con- 

 nected by radiant rows of minute granules, with an umbilical rosette 

 of oblong cellules. Kecent, New Zealand and Monterey Bay ; 

 fossil Monterey Stone. An elegant species distinguished by its 

 somewhat mammiform processes, which, being directed outwards, 

 appear lunate. Granules punctiform, proceeding from umbilicus 

 to processes in pencil-like rays ; intervals between the processes 

 bisected by similar pencils but less conspicuous, and without 

 furrows ; the rest of the granules in oblique lines, as in A. Petersii." 

 This description is not to my mind very accurate ; the granules 

 certainly appear to run in places in oblique lines, and in others in 

 curves, and the lines and curves are decussating (which he does 

 not notice), but all are equally radiant from the centre to the 

 margin ; and all the lines of granules, except the two which run 

 on each side of the rays from the centre to the processes are 

 branched before they reach the margin. I believe the fact to be as 

 Mr. Hardman describes it in a letter I received from him yester- 

 day. He says — " The lines of granules I believe invariably 

 bifurcate ; indeed, they must of necessity do so if the space between 

 the granules be alike and the granules all of one size — which, I 

 think, you will find they are, except the granules of the ' primary 

 rays.' Although it may be said that granules run obliquely from 



