1 52 THE VOYAGE OF H.M.S. CHALLENGER. 



Coscinodiscus, Ehrenb. 



Among the numerous different forms of Diatoms which have hitherto been recorded, 

 those possessing discoidal outlines are without doubt the most elegant in appearance, but 

 they are at the same time the most rarely met with. Thus among freshwater forms they 

 are typified by the genus Cyclotella and a few Melosirse, while in marine collections from 

 the sea-shore or from the washings of sea- weeds they are only found in rare cases. To Ehren- 

 berg is due the credit of having first directed the attention of naturalists, geologists, and 

 microscopists to the so-called Infusorial Earths, tripoli and other siliceous deposits formed 

 by the accumulation of myriads of the siliceous remains of Diatoms that lived in bygone 

 geological eras. Many of these deposits manifestly represent the bottoms of ancient seas 

 which have been elevated by subterranean forces, and the examination of the organisms 

 which they contain has revealed many new diatomaceous forms, among which discoidal 

 frustules more beautiful and elegant than any before known, occur in great numbers. The 

 principal type of these discoidal forms is to be found in Coscinodiscus, Ehrenb., a genus which 

 has been defined by Pritchard (History of the Infusoria, p 1 827) in the following manner : — 

 "Frustules single, discoid; disc cellular or dotted, without processes, defined border, internal 

 septa, or division into radiating compartments." Hence in brief any simple cellular or 

 punctated disc is called a Coscinodiscus, while the character of this cellulation or punctation 

 — whether strongly marked or minute, whether increasing from the margin to the centre or 

 conversely, whether regular or without order, whether in linear or curvilinear arrange- 

 ment, whether radiating or excentric, or forming a rosette or umbilicus — constitutes the 

 differential characteristic of many species. 



That the extension of research in this department of marine biology should result in the 

 continued increase of the number of species of this important genus is not to be wondered at, 

 and the collection procured by the Challenger Expedition is a large and interesting one. 

 Three magnificent new species which were brought home by this Expedition have been 

 already described by the well-known Irish microscopist, Rev. E. O'Meara, M.A., and 

 to the first of these the name of Coscinodiscus craspedodiscus, O'Me., 1 has been given 

 on account of its very great size (Plate III. fig. 5). It may readily be seen with the 

 naked eye, presenting the appearance of a hoop or ring, one millimetre in diameter. Its 

 hoop-like form is due to the circumstance that the outline is somewhat convex and strongly 

 siliceous, with large hexagonal areolae, while the central part is very thin and so trans- 

 parent that an accurately adjusted illumination is required to reveal its sculpturing and to 

 discover the form of its smooth central areola. The second has been named Coscinodiscus 



1 This interesting Diatom has been defined as follows: "Diameter 0O22", centre large, free from arcola- 

 tion. Areoles radiate, at the margin large, hexagonal, thence somewhat compressed, decreasing in size towards 

 the centre, and somewhat elongated. Towards the centre some of the radiate lines of areoles are somewhat 

 shorter than others, in consequence of which the free centre has somewhat of a star-like appearance." — Quart. 

 J own. Mia: Sci., vol. xvii. p. 5G1 



