Vol. 3] Kofoid. — Triposolenia. 99 



The transverse furroiv {tr.f. PI. 16, fig. 6) encircles the head 

 as a Avide equatorial band, which is slightly if at all impressed. 

 Its floor is flattened in T. bicornis (PI. 15, fig. 2), slightly con- 

 cave in T. depressa (PI. 16, fig. 3), or convex in T. ramiciformis 

 (PI. 17, fig. 7). There is no appreciable displacement distally in 

 the fnrrow, that is, it does not form a spiral abont the head. 



The anterior list or collar {a.l. PI. 16, fig. 6) is a flaring hya- 

 line sheet with 12-20 equidistant ribs. There are two prominent 

 ventral ribs at the ventral suture. The plane of both lists is 

 oblique to the neck, being directed ventro-posteriorly, slightly 

 beyond the perpendicular to the axis of the midbody. The an- 

 terior list flares obliquely anteriorly at an angle of 10° or more 

 and is frequently concave posteriorly. On account of the poste- 

 rior displacement of the flagellar pore the posterior list {p.l. PL 

 16, fig. 6) of the transverse furrow has an unusual form in both 

 Triposolenia and Amphisolenia. 



Instead of forming a simple circle about the girdle as in Peri- 

 dinium, the posterior list turns abruptly posteriorly at the ven- 

 tral suture and continues posteriorly along the ventral side of 

 the neck in two parallel wings to the flagellar pore forming the 

 pseudo-longitudinal furrow. The transverse flagellum {tr.f I- PI. 

 16, fig. 6) runs anteriorly to the collar in this channel. The right 

 list is not so high as the left and both rise from the thickened 

 ridges on the neck. Posteriorly the two fins diverge to enclose 

 the flagellar pore [f.p. PI. 16, fig. 6) which lies in the pharyngeal 

 enlargement. The height of the left fin increases at the pore and 

 it swings in toward the median line behind the pore. In the re- 

 gion of the sagittal plane there is usually present a suture spine 

 {s.sp. PI. 16, fig. 6) beyond which the fin continues posteriorly 

 for a short distance along the anterior process, and dies out with- 

 in a short distance of the pore. This spine is doubtless homolo- 

 gous with the midspine of the ventral fin or left longitudinal list 

 of Dinopliysis, which Stein ('83 Taf. 20, fig. 19) has shown to be 

 located near the suture line in Dinophysis. It appears from 

 Stein s figure and from other evidence in my hands not yet pub- 

 lished, that the ventral fin in the Dinopkysidae lies on the left 

 valve anterior to the midspine and on the right valve posterior 

 to this spine. In some genera this spine is frequently doubled. 



