REPORT ON THE DIATOMACE.E. 149 



the different valves the ventral line and margin alternately were found sometimes to be 

 provided with, but sometimes to be devoid of, a nodule ; so that in such forms the 

 superior valve has not the marginal nodule which exists in the inferior. This circum- 

 stance recalls what takes place in Cocconeis and Achnanthes, since in the former a central 

 nodule, and in the latter a stauros, is to be found only on the inferior valve. 



Since, then, the genus Euodia, Bail., was instituted prior to that of Hemidiscus, 

 Wall., the name of the united genera must be Euodia, Bail., and its definition may 

 be amended as follows : — Frustulum cellulosum vel granulatum ; valvis lunatis ; nodulo 

 ventrali maranali in inferiori valva. 



I have already had the opportunity of observing similar lunate valves while preparing a 

 report on a sounding made in the Atlantic by the "Porcupine" in 1869. * At that 

 time I noted the occurrence of lunate valves with and without the central nodule, and 

 possessing profiles and structural characteristics sufficiently distinct from those of Hemi- 

 discus cuneiformis, Wall., 2 by the absence of the marginal series of points and the much 

 oreater delicacy of the radiating granulation, to justify the establishment of a new species. 

 I then had a presentiment, which has now been verified by actual observation, that the 

 two lunate forms, the one with and the other without the nodule, should be indicated as 

 the inferior and superior valves of the same species ; and it may be well here to append the 

 definition of the species then established which is reproduced on Plate XII. fig. 1, so as 

 to rectify the generic name, and to note the presence of a nodule on the inferior valve : — 

 Euodia (Hemidiscus) inornata, Cstr. — Valvis lunatis ; linea dorsali parabolica, 

 ventrali late arcuata ; punctulis vel denticulis marginalibus nullis ; nodulo marginali in 

 inferiori valva. In mari Atlantico Septentrionali et in Oceano Pacifico. 



Euodia recta, n. sp. (Plate XII. fig. 3.) 



E maximis ; linea dorsali late arcuata, ventrali recta ; cellulis subradiantibus ; apicibus 

 acuto-rotundatis. In Oceano Atlantico meridionali. 



Among the Euodise brought home by H.M.S. Challenger this form is distinguished by 

 its singular outline and its large dimensions. It is 132 m in diameter, and its straight 

 ventral line resembles the chord of an arc formed by the curved dorsal margin. The 

 extremities are acutely rounded, and the sculpturing is cellulate and subradiating. 



The specific name of this Atlantic species has reference to the character of the ven- 

 tral side, 



Euodia orbicularis, n. sp. (Plate XII. fig. 15.) 



Forma minima, inflata ; linea dorsali curvata, ventrali arcuata ; apicibus obtusis ; 

 minimis punctulis conferta. In Oceano Atlantico meridionali. 



1 Atti Accad. Pontif. d. nuov. Lincei, March 1871. 



2 Micr. Joum., vol. viii. p. 42, pi. ii. figs 3 and 4. 



