28 THE VOYAGE OF H.M.S. CHALLENGER 



This elegant naviculoid frustule, found in the interesting collection from the port 

 of Tahiti, recalls, when viewed in profile, the Navicula proserpinae of Ehrenberg, 1 

 which is identical with the Navicula kutzingii 2 of Grunow. Its two lobes are sub- 

 cordate and the extremities are cuneato-acute. The nodule, which in Ehrenberg and 

 Grunow's type is large and quadrate, is here very small, nor does the frustule present such 

 longitudinal bands as separate the strise of the Navicula proserpince and Navicula 

 kutzingii. 



In Navicula thaitiana the radiating moniliform strise gradually become less visible 

 towards the centre. 



Navicula entomon, Ehrenb., var. thaitiana, nov. (Plate XX. fig. 17.) 



We have here represented a frustule which is probably only a variety of Navicula 

 entomon, 3 Ehrenb. Its centre is more contracted than that of the typical species, and its 

 two lobes present an elegant elliptical profile. Both forms possess lines of slightly sub- 

 radiating but manifest granules, which differ from those of all other bi-lobed navicular 

 frustules. The present diatom has a very strong round central nodule, but this charac- 

 teristic, taken by itself, cannot be regarded as sufficient to constitute a species. 



Navicula entomon, Ehrenb., var. (?) (Plate XX. fig. 10.) 



This frustule can scarcely be said to be sinuately constricted, but its raphe, which is 

 slightly bent when viewed in profile, really possesses a double curvature and is flanked 

 on both sides by a band of granules. 



With reference to these peculiarities it is to be remarked that they are probably merely 

 developmental stages in the life-history of the species. Thus in the younger frustule the 

 raphe may be straight instead of bi-curved, while in embryonic phases the granules are 

 not well defined but become more and more marked as age advances. 



Navicula grunowii, Rabenh. (?) (Plate XV. fig. 9.) 



The present figure represents an elegant Navicula possessing a valve which is almost 

 linear in profile. On comparison with Plate lxx. of Schmidt's Atlas it is found to coin- 

 cide almost exactly in its characteristics with Navicula pristiophora, Janisch, and with 

 Navicula grunowii, Eabenh. 4 It may accordingly be regarded as intermediate between 



1 = Diploneis proserpince, Ehrenb., Monatsber. d. k. Akad. d. Wiss. Berlin, 1858, p. 14; see Pritcbard, 

 loc. cit., p. 893. 



2 This form is shown in Plate xiii. fig. 24 of Schmidt's Atlas ; see also Verhandl. d. k. k. zool.-bot. Gesellsch. 

 in Wien, 18G0, p. 532, T. i. fig. 15. 



3 Established by Ehrenberg in 1840; = Pinmdaria entomon, Ehrenb. (Verb. T. i. 1, P. 3 et 4 ; Kiitz., Bac., 

 p. 100, T. xxviii. fig. 74); and = Diploneis entomon, Ehrenb. Mikrogeol., T. xix. f. 30; see also Pritchard, loc. 

 cit., p. 893. 



4 See Schmidt's Atlas, pi. lxx. figs. 72 and 73; and Rabenhorst's Flora Europaaa Akjarum, sect. 1, p. 203, 

 Lipsise, 1864. 



