"^^^^ 3] Kofoid. — Triposolenia. 95 



In PJialacroma, Onuthocercus, and Histioneis they consist merely 

 of reticular extensions from the thecal wall. In the two genera 

 first named, however, the thecal wall itself and the protoplasmic 

 contents are prolonged posteriorly in well defined antapical horns, 

 one in Amphisolenia and tM^o in Triposolenia. 



The pair of processes in Triposolenia exhibits certain constant 

 relations which give to the symmetry of the organism a tripartite 

 character that is unique among the Dinophysidae, though forms 

 exhibiting a superficial resemblance are seen in the genus Cera- 

 tium and in Peridinium. In both of these genera, however, the 

 antapical horns are right and left, that is, the bifurcation is bilat- 

 eral, while in Triposolenia the horns are dorsal and ventral, and 

 the bifurcation is sagittal. The only other instances of sagittal 

 bifurcation among the Dinoflagellates are found also in the family 

 Dinophysidae, in Amphisolenia hifurcata (Murray and Whitting, 

 '99), A. tJtrinax (Schiitt, '93), and A. quinquscauda, and in 

 Phalacroma idtinia. The bifurcation is, however, but feebly ex- 

 pressed in these species as compared with its development in Tri- 

 tposolenia. The tripartite character of this genus is shown in the 

 proportionate development of the anterior process and the pair 

 of antapical horns. The distance of the apex of the epitheca from 

 the center of the midbody is approximately equal to that of the 

 antapices of the horns from the same point. The proportionate 

 development of the two antapicals and the anterior process is 

 equally evident in the short-horned forms of the San Diego re- 

 gion, and the long-horned ones of the warmer waters of the 

 tropics. Elongation of the anterior process is coincident with 

 elongation of the antapicals and is present throughout all types 

 of curvature and origin of the horns. In species with antapicals 

 arising from the jjosterior angles of the midbody this equality is 

 less marked than in those with the pendant midbody, the anterior 

 process being a trifle shorter in the first named group. The ven- 

 tral horn is also a little longer than the dorsal in all the species, 

 an asymmetry which finds a parallel in the uniformly shorter 

 right horn of the bifurcated species of Ceratimn and Peridinium. 

 The ventral antapical of Amphisolenia and the right one of Cera- 

 tiurn and Peridinium lie on the side nearest to the longitudinal 

 flagellum and to the axis of rotation and of locomotion. Elouga- 



