166 THE VOYAGE OE II. M.S. CHALLENGES. 



This genus has been named in honour of Dr Rudolph von Willemoes-Suhm, one of 

 the naturalists on hoard H.M.S. Challenger, who died during the Expedition. 



A more precise statement of generic and specific characters may in the meantime 

 be omitted pending a further series of observations on this very interesting group of 

 organisms. 



Ethmodiscus, 1 n. gen. 



Frustula solitaria, discoidalia ; valvis tenuissime et inconspicue striolatis ; forma plus 

 minus convexa, quandoque diversimode denticulata ; zona connectiva punctulata. 



Under the genus Coscinodiscus, which is well adapted for the reception of species 

 having conspicuously granulated markings, the desirability of establishing a new genus 

 for certain forms possessing an almost invisible sculpturing was alluded to. For this new 

 genus the name of Ethmodiscus is here adopted, a word which has reference to the 

 presence on the valves of extremely delicate points, which may be compared to the fine 

 pores of a filter. Although such forms could not have been accurately observed by the 

 use of the relatively imperfect microscopes employed by Ehrenberg, and although it is 

 with the greatest difficulty and by the use of the greatest obliquity of illumination that 

 the best instruments of the present day can reveal the exquisite delicacy of their 

 ornamentation, yet the opportuneness of establishing the genus has been proved by the 

 discovery in surface gatherings of certain discoid forms of enormous size and extreme 

 fragility, which are characterised by their very minute granulations. 



The form of the valves of the specimens bearing these delicate markings — which cannot 

 be made out when they are mounted in balsam, but become visible in dry preparations — 

 is found, when they are broken by the action of the flame of a spirit-lamp, to be notably 

 convex. 



The existence of large fragments of frustules, which must also have belonged to 

 Diatoms of extraordinary size, and which bear characteristic punctations (Plate XIV. figs. 

 4 a, 4 b, 4 c), had often been revealed by the examination of organic siliceous forms derived 

 chiefly from soundings. These fragments, however, remained undetermined, and their 

 true significance has only recently been disclosed by preparations made on board the 

 Challenger. One of these preparations includes three large cylindrical Diatoms terminated 

 by two notably convex surfaces (Plate XXII. fig. 10), and provided with hoops or con- 

 necting zones showing punctations arranged in a quadrate manner. This would lead one 

 to believe that the fragments in question belonged to the connecting zones of Diatoms of 

 this genus ; and this belief has now been proved by the recognition in one of these 

 fragments of converging lines of small points, along with a similar quadrate sculpturing 

 on a portion adhering to its margin. The difficulty of finding any converging lines or any 



1 qt)/j.o(, a sieve. 



