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



from Biddulphia; while, like Zygoceros, Porpeia differs from it merely by livincr in an 

 isolated condition. In Amphitetras the valves are subquadrate, and provided with four 

 pseudo-openings at the angles, while in Biddulphia there are only two. Yet the struc- 

 ture of both of the latter types is the same, while the number of pseudo-openings depends 

 on the form of the valve — there being one at each angle ; thus some triangular forms, or 

 forms with four or more angles, each provided with a pseudo-opening, occur, and have 

 hitherto been named Triceratium, but these must in future be enrolled among the Bid- 

 dulphia. Amphitetras agrees with Biddulphia in its zigzag appearance, the union 

 between its frustules being effected by means of an isthmus or small angular cushion ; 

 and, in its structure, it agrees with the latter so far as to possess a pseudo-opening at 

 each extremity. Moreover, Amphitetras antediluviana, Ehrenb., 1 i3 frequently found 

 associated with the Biddulphia pulchella of Gray, which is but another proof of the 

 affinity of the two types. 



Biddulphia pulchella, Gray, var. major, nov. (Plate XXIII. fig. 6.) 



We have here represented a very large valve divided by six transverse lines or septa, 

 the characters of which exactly agree with the definition of the Biddulphia pxdchella of 

 Gray, except that the strong thorns, which are constantly found in groups of two or three 

 in the latter, are absent in the former. For this reason, the present form has been referred 

 to as a variety, a view which is, in part, substantiated by its unusually large size. It was 

 collected in the Caroline Archipelago. 



Biddulphia reticulata, Roper, var. inermis, nov. (Plate XXVI. fig. 9.) 



Another beautiful form of this geuus is here shown. It possesses reticulated parietes, 

 and two opposite capital processes, towards the extremities of which the network dis- 

 appears. Between the two latero-terminal processes the end of the frustule is slightly 

 inflated or convex. In these respects it resembles Biddulphia reticulata, Roper, 2 but it 

 may be distinguished from the latter by the absence of superficial puncta. 



It may be observed that in the present figure the two frustules are united together by 

 means of two hoops or belts, which embrace each other, and each of which shows a dif- 

 ferent internal structure from that of the valve. This appearance enables us to understand 

 the transitory nature of the belt in some forms, as has been already pointed out by Dr 

 Wallich, who observed that it did not form an integral part of the frustule, but was joined 

 to it by means of several distinct oblong cellules. 



This elegant organism was collected at the Philippine Islands. 



1 Pritchard/o/i. cit., pi. xi. figs. 21 and 22. 



2 Trans. Micr. Soc. Lond., vol. vii. p. 14, pi. ii. figs. 13-15. 



