170 THE VOYAGE OF H.M.S. CHALLENGER. 



thereby showing the uncertainty which must often exist in the definition of species when 

 these have not been established upon living forms. Its valves and connecting zone 

 exhibit a slight striation. 



Ethmodiscus wyvilleanus, n. sp. (Plate XIV. fig. 6.) 



E maximis ; subcylindricum ; valvis convexis medio complanatis, radiolatis ; zona 

 connectenti punctulata. Diametrum tequatoriale = 1000 m. : polare = 1457 m. In Oceano 

 Atlantico. 



Although smaller than Ethmodiscus gigas, this species is still of colossal size. Its valves 

 are convex, and each has a flat slightly compressed centre. The connecting zone is greatly 

 developed, so that the frustule is cylindroidal, the ratio of the equatorial to the polar axis 

 being approximately as 2 to 3. This zone is clearly punctated in a quadrate manner, and 

 the valves are ornamented with radiating strise or rows of small points. 



Ethmodiscus tympanum, n. sp. (Plate XIV. fig. 3.) 



E maximis ; exacte cylindricus ; valvis annulatis cingulo inclusis. Ad superficiem 

 maris Japonici. 



This curious type is not so large as the two preceding. Its form is exactly cylin- 

 drical. It possesses, like Biddulphia, a distinct belt superposed to the hoops of the two 

 valves. 



The specific name has reference to its cylindrical outline. 



Ethmodiscus (?) sphaeroidalis, n. sp. (Plate XXII. fig. 10.) 



Frustulis spherico-compressis, binatim conjunctis; valvis convexis inaequalibus, una 

 scilicet convexo-complanata, altera convexa et medio concava. Diametrum = 882 At. In 

 mari Pacifico. 



This species is interesting not only on account of its size, but also because two simi- 

 lar frustules were found associated together — a circumstance which renders the generic 

 determination doubtful. Both forms are of a compressed spheroidal outline and are 

 united by a common belt, which exhibits a very delicate line of suture in the middle. 

 This belt is somewhat analogous to that which occurs in some Melosine and Podosirie. 

 On each frustule two other lines, which probably indicate the incapsuling of the two 

 valves, occur. 



It is to be noted that, although the character of the belt seems to indicate a relation- 

 ship to the Melosirse and Podosirse, its size and fragile nature point to an affinity with 

 Ethmodiscus. 



Plate XIV. figs. 4, 4a, ib, 4c, represent fragments of various species. 



