"276 Proceedings of the Royal Irkli Academy. 



here, altliougli differently arranged by otlier antliors. liVni. Smith, 

 recognising the affinity between Biddnlphia and Ampliitetras, placed 

 them close together, bnt assigned to Triceratium a widely different 

 position in his system of arrangement. Kiitzing distributes the in- 

 cluded genera in two distinct groups — the Anguliferfe and Angulatge, 

 between which he interposed the Biddulphieas and Tripodiscus argus 

 = Eupodiscus argus. The Anguliferte, he says, "are easily distin- 

 guished by means of their angular side view ; " but of the Angulatfe, 

 which embraces only the single genus Triceratium, he gives no other 

 diagnosis than that contained in the description of that genus, "indi- 

 viduals free, with the bivalve lorica triangular, not concatenated," 

 Ealf s omits TiTaacria, a genus established by Heiberg subsequently to 

 the publication of the '' History of the Infusoria," and along with the 

 other genera placed in the present group includes Euodia, and Hemi- 

 discus. Of Hemidiseus I have never seen a specimen, and, therefore, 

 ■can express no opinion regarding it ; but as to Euodia, from the cuneate 

 •outline of its transverse section, it plainly should be excluded from this 

 group, with which it has little, if any, affinity. Ralfs indicates two 

 features by which the fonns in this group may be distinguished from 

 the Bidclulphieae : " The angles on the fi'ont view are usually less 

 elongated, and the intervening margin less lobed." Of these charac- 

 ters the latter can scarcely be sustained in all cases ; and as to the 

 former, if Trinacria, in which the processes at the angles are veiy 

 long, is to be admitted here, this, too, must be regarded as by no 

 means a satisfactory diagnosis. Grunow docs not refer to Trinacria, 

 for the same reason as Ralf s, but includes the other forms of this group 

 under the Biddulphieae, which he thus defines : ' ' Yalves on side view 

 longish, or three, foui', or more angled," no reference being made to the 

 processes springing from the angles which constitute so remarkable a 

 feature of these forms. According to this author, the characteristic 

 distinction between Amphitetras and Triceratium is the possession of 

 four angles by the former, while the latter has but three. The fact 

 that specimens of the former occur with five angles, and of the latter 

 with four or more angles, evinces how untenable is this distinction as a 

 generic diagnosis. 



Heiberg includes Amphitetras and Triceratium in the Biddulphieas, 

 and his genus Trinacria in another group, namely, the Hemiaulidas ; 

 the main distinction of which rests on the form and position of the 

 processes, which are triangular, and spring at right angles fi'om the 

 basal plane of the valve. But these cliff erences, though sufficient to 

 establish generic distinction, seem scarcely to justify the establishment 

 of a distinct group to receive the forms. It will thus appear that, in 

 ■consequence of the projection of the processes fi'om the angles of the 

 valves, the relationship of this group to the Biddulphiece is recogiiised 

 by most authors : but no more satisfactory distinction between Am- 

 phitetras and Triceratium has been suggested than that in the former 

 the frustules are concatenate, and in the other fi'ee. This distinction 

 I adopt, not because I consider the supposed fact on wliich it rests in 



