Norman, on Diatomacea. 9 



Hab.— Stomach of Ascidians, Shark Bay, western coast 

 of Australia ; kindly commnnicated by Dr. Macdonald. 



This singularly beautiful form is exceedingly rare in the 

 above-mentioned gathering. It belongs unquestionably to 

 the group of forms of which New. lyra is the type. The re- 

 markable row of bosses on the smooth bands renders it 

 distinct from any known species. 



It may be remarked that, in describing the structure of 

 Coscinodiscus fuscus and Aulacodiscus Sollitiianus, I have 

 designated the markings on the valves " granules,^^ instead 

 of adopting the usual method of calling them areolae, or cells. 

 Hitherto, I believe, most authors have adopted ihe latter desig- 

 nation in describing the various species of Coscinodiscus, 

 Aulacodiscus, Eupodiscus, &c., and have, by so doing, in my 

 opinion, overlooked the real nature of the construction. 



Dr. Wallich has done good service in pointing out the true 

 structure of the markings of Pleurosigma, and I feel con- 

 vinced that all the above-mentioned discs are constructed on 

 the same plan, differing only in the form of the elevations or 

 granrdes, and their arrangement on the surface of the valve. 



In Pleurosigma the markings are four-sided elevations; 

 while in Coscinodiscus, Eupodiscus, &c., they are circular 

 when not crowded, but assuming the irregular or hexagonal 

 form when pressing on each other. The same structure ap- 

 pears to exist in Biddulphia, Isthmia, &c., and probably in 

 all diatoms, not even excepting Triceratium favus, the raised 

 portions of silex only differing in form. 



On examining a valve of Coscinodiscus gigas, or lineatus, 

 for instance, with a good one fourth or one twelfth, we find 

 the colour of the valve, in the interstices between the granules, 

 to be pink, whereas the granules themselves are white, or 

 colourless. 



The true structure, however, is better seen in valves 

 where the granules are more circular, and not so much 

 crowded together. Here the structure will be apparent at a 

 glance. 



