REPORT ON THE RADIOLARIA. 1193 



The family Tripocyrtida, composed of the Sethopilida and Sethoperida of my 

 Prodromus, comprises those Cyrtoidea in which the lattice -shell is two -jointed and 

 bears three radial apophyses. The two subfamilies differ in the shape of the mouth, 

 which in the Sethopilida is a simple wide opening, but in the Sethoperida is closed 

 by a lattice-plate; the former are here divided into sixteen, the latter into eight 

 different genera. Though probably the two shell -joints are not truly homologous in 

 all Tripocyrtida, we call the first joint here, as in all Dicyrtida, the cephalis, and 

 the second joint the thorax. 



Numerous Tripocyrtida, living as well as fossil forms, were formerly described by 

 Ehrenberg. His genera Dictyophimus, Clathrocanium, Lithomelissa, and Lychno- 

 canium belong to the Sethopilida, and have the mouth open; his genera Lithopera and 

 Lithochytris (partly) belong to the Sethoperida, and have the basal mouth closed by a 

 lattice-plate. Many of these Tripocyrtida belong probably to the oldest forms of 

 Dicyrtida, are nearly related to the Phormospyrida, and therefore of special phylogenetic 

 interest, as was demonstrated by Biitschli (1882, loc. cit., pp. 514519). This 

 near relation to certain Spyroidea (Tripospyris, Acrospyris, &c.) is particularly 

 striking in some forms of Clathrocanium, Lithomelissa, &c. Some other Tripocyrtida 

 seem to possess a closer relation to certain Plectoidea (Plagoniscus, Plectaniscus], 

 so mainly some forms of Tripocyrtis and Dictyophimus. 



The cephalis, or the first joint of the shell, corresponds usually to the whole shell 

 of the Zygospyrida and of numerous Monocyrtida, and exhibits various modifications 

 of shape, which have been already described in these latter families. It is usually 

 subspherical or hemispherical and armed with an apical horn. In a small number 

 of genera the horn is lost, in some other genera multiplied. The cephalis is separated 

 from the thorax not only externally by the collar constriction, but commonly also 

 internally by a transverse horizontal fenestrated septum, which usually exhibits three 

 or four characteristic cortinar pores. The central capsule, originally enclosed in the 

 cephalis, develops usually three or four large pear-shaped csecal sacs which pass through 

 the cortinar pores and depend into the thorax (PI. 55, figs. 211 ; PI. 60, 

 figs. 3-7, &c.). 



The thorax in this family exhibits a great variety of interesting modifications, 

 mainly in the development of the three radial apophyses arising from it. These may 

 be either enclosed in the wall of the thorax as ribs, or arise as free wings, very often 

 prolonged over the mouth as three terminal feet. Finally the three terminal feet only 

 remain, whilst the original ribs are lost. The special ornamentation of these three 

 apophyses exhibits an extraordinary variety and elegancy of structure, and many Tripo- 

 cyrtida belong, no doubt, to the most graceful and admirable forms of NASSELLAEIA. 



(ZOOL. CHALL. Exp. PART XL. 1886.) Rr 150 



