98 University of California Puhlications. [Zoology 



axial length. Its form varies much in the different species. It is 

 laterally compressed and its transdiameter is not much greater 

 than that of the neck. Its dorso-ventral diameter is more ex- 

 tended and its anterior face is rounded in most species, but con- 

 cave in T. truncata (PI. 16, fig. 5). 



The antapical liorns {a.h. PI. 15, fig. 2) are latero-posterior 

 extensions from the midbody, of approximately circular cross- 

 section, and curved from a lateral direction at their origin to a 

 posterior one distally. The dorsal horn is always a little shorter 

 than the ventral and the two horns stand in a balanced relation 

 to the axis of the midbody. The curvature is in some cases local- 

 ized in a major flexure (wi./. PI. 17, fig. 7) and in others, as in 

 T. truncata, is distributed. The distal ends of the horns exhibit 

 in lateral view in some species a slight sigmoid curvature, the 

 antapices being deflected outward. The tips are without spinules 

 and rounded (T. exilis PL 17, fig. 8), or truncate with dorso- 

 ventral spinules (T. truncata PI. 16, fig. 5), or with lateral spin- 

 ules (T. depressa PI. 16, fig. 3). In all species but T. truncata 

 the outer margins of the major flexures or adjacent regions bear 

 2-4, rarely 1-2, or 5-7 small elevations or tubercles {t. PI. 15, 

 fig. 2) , each pierced by a pore. They lie close to the sagittal suture 

 and are conical elevations deflected posteriorly. In addition to 

 the curvature in the dorso-ventral plane there is in all species a 

 slight bilateral asymmetry, in that the antapical horns do not lie 

 in the sagittal plane of the midbody but are bent to the right 

 slightly in the middle or anterior regions and distally to the left, 

 the dorsal horn exhibiting more of the distal curvature than the 

 left. 



The tJiecal wall in this genus, as also in most species in Am- 

 phisolenia, is hyaline and apparently structureless. It is of 

 nearly uniform thickness except on concave faces of the horns, 

 where, as in Ceratium, the wall is somewhat thickened, especially 

 in the more robust individuals. Under the highest magnification 

 the wall is very faintly and minutely spotted or flecked, as if in 

 the incipient stages of pitting or reticulation. In but a single 

 species, T. truncata, are these structures developed into plainly 

 visible pits and reticulations. The pores are found only in the 

 tubercles and along the ventral suture on the neck. 



