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



On Plate XXVI. fig. 10, there may be seen Biddulphia tuomeyi, Bail., 1 and on Plate 

 XXX. fig. 6, a variety of this — Biddulphia tuomeyi, Bail., var. pacifica, nov., from the 

 Pacific Ocean. 



Biddulphia roperiana, Grev., 2 is represented on Plate XXVI. fig. 4. 



Triceratium, Ehrenb. 



Soon after Ehrenberg established the genus Triceratium among the anguliferous Diatoms, 

 frustules- were found which exactly corresponded to its essential characters, but which had 

 valves with four or five angles, and these frustules were admitted into the genus as excep- 

 tional forms — Triceratium being defined as possessing " valves rarely with four or five 

 angles." s Later on, frustules with six, seven, and even twelve angles had to be enrolled, as, for 

 example, the Triceratium eulensteinii of Grunow (see A. Schmidt, Atlas, Plate lxxv. fig. 6). 

 Although this last species is probably a real Stictodiscus, it can hardly be doubted that, 

 if Ehrenberg had foreseen the extension of his genus to embrace polyangulated frustules, 

 he would have chosen the name Polyceratium. The following is the definition of the 

 genus Triceratium as given by Pritchard : — " Frustules cellular, free, simple, in lateral 

 view triangular (rarely with four or five angles)." 



More than 200 species have been ascribed to this genus, about 150 being figured in 

 the Microscopical Journal alone. It is to be remarked, however, that many of these so- 

 called " species " have no real claim to be regarded as true specific types, and several forms 

 which have been observed amongst the Challenger material have been relegated to poly- 

 gonal types of Stictodiscus, although they might at one time have been placed in the 

 present genus. 



Triceratium pulvillus, n. sp. (Plate VI. fig. 8.) 



Valvis quadratis, areolatis vel cellulatis ; laterales linese late concavse ; apices acuto- 

 rotundati ; areolae aequales, hexagonales ; processu terminali nullo. Ad mare Philippi- 

 narum. 



This quadrate frustule from the Philippine Sea possesses greatly hollowed-out sides and 

 acute rounded extremities. The whole surface is occupied by equal areolae or hexagonal 

 cellules, there being no trace of any process on the extremities. 



Triceratium thaitiense, n. sp. (Plate XIII. fig. 14.) 



Hexagonum, lateribus concavis, apicibus rotundatis ; valva punctulis rariusculis signata. 

 In portu Thaiti. 



This hexagonal form was observed in a collection made with a tow-net, at a small 

 depth in the port of Tahiti. It is provided with concave sides and rounded extremities, but 



1 = Zygoceros tuomeyi, Bail., Amer. Journ., vol. xvi. pi. iii. figs. 3-9; Pritchard, op. cit, p. 848, pi. vi. fig. 10. 



2 Micr. Journ., vol. vii. pi. viii. figs. 11-13. 3 Pritchard, op. cit., p. 853. 



