FAMILY PERIDINIACEAE 



19 



16). The first girdle plate (Ig) is a short plate, about 5 

 microns long, at the proximal end of the girdle, where 

 it borders the anterior plates of the ventral area. The 

 second girdle plate (2g) extends from Ig between plates 

 Ipr and Ipo, as far as the lateral edges of the latter 

 plates, so that the suture 2g/3g is continuous with the 

 sutures lpr/2pr and lpo/2po. The third girdle plate 

 (3g) is very long and runs completely around the dorsal 

 side of the body to a corresponding suture on the right 

 ventral side at the lateral edges of plates 7pr and 7po. 

 The fourth girdle plate (4g) runs from 3g to the right 

 side of the sulcus forming the distal end of the girdle. 

 The skeletal structure of the ventral area is very 

 complex as compared with the skeletal composition of 

 the rest of the body. Externally the sulcus is composed 

 of five plates (figs. 17, 18; pi. 1, figs. A, B). The ante - 

 rior sulcal p late (a.s.) is at the right anterior end of the 

 ventral area. Its anterior end runs slightly into epithe- 

 ca above proximal end of girdle; posteriorly it expands 

 and forms the anterior edge of flagellar pore. The left 

 sulcal plate (l.s.) composes most of left side of sulcus. 

 Its right edge forms the left edge of flagellar pore. The 

 posterior sulcal plate (p.s.) is a narrow, roughly U- 

 shaped plate constituting posterior end of sulcus. The 

 right sulcal plate (r.s.) occupies right side of sulcus and 

 its left edge forms right edge of flagellar pore. 



From both the anterior and posterior edges of the 

 flagellar pore, there extend into the cell body two inter- 

 nal processes, the anterior pore process (a. p. p.) and the 

 posterior pore process (p. p. p.). These processes (figs. 

 17B, 18A, B) are curved in cross section so that they 

 form grooves running into the cell body, apparently 

 forming supporting structures for the two flagella which 

 emerge from the cell body at the flagellar pore. The 

 anterior pore process is constructed principally of an 

 internal extension of the posterior edge of the anterior 

 sulcal plate. On the right side, it is tied to the right 

 sulcal plate by the right internal sulcal plate (r.l.s.), 

 which runs along the entire right edge of the process, 

 and posteriorly along the left interior edge of the right 

 sulcal plates, and part way along the posterior pore 

 process. Along its entire course it is underlaid by a 

 membrane (m) attached to its adjacent plates. The left 

 side of the anterior pore process is formed in part by 

 the anterior groove list (a.g.l.), which also extends part 

 way along the external portion of the anterior sulcal 

 plate. The groove in the posterior pore process, as it 

 comes to the surface, runs posteriorly to the end of the 

 sulcus. The internal portion of this groove, i.e., the 

 posterior process, is formed on the right side princi- 

 pally by an extension of the right sulcal plate. The pos- 

 terior and left sides are formed by a separate plate, 

 the posterior accessor y sulcal plate (p. a.s.). Where the 

 posterior groove runs along the exterior, however, it is 

 overlaid by a listlike process originating from the sides 

 of the posterior and right sulcal plates, the p osterior 

 groove list (p.g.l.). The exterior extension of the pos- 

 terior groove is open on the left side but covered ven- 

 trally by this list (figs. 17, 18C; pi. 1, figs. A, B). 



The right edge of the flagellar pore is bounded by 

 the right accessor y sulcal list (r .a.s.), which is attached 

 along the edge of the right sulcal plate. In its posterior 

 portion it joins with the posterior groove list. 



A cross section of the right sulcal plate (fig. 18D) 

 shows the curvature of that plate as well as the relative 

 position of the right accessory sulcal list and the mem- 

 brane to which the right internal plate attaches. 



Thecal wall . Surface of plates sculptured with ir- 

 regular reticulation which is difficult to demonstrate, 

 except at intersections, where it is raised into irregu- 

 lar points. 



Pores occur throughout all major plates, irregu- 

 larly scattered and bearing no relation to reticulations; 

 absent in girdle plates. 



Boundaries between plates vary from simple su- 

 tures to wide intercalary zones. Rabbet joints present 

 in all parts of theca (see p. 10). Intercalary zones and 

 rabbet membranes occur in the girdle at the 2g/3g and 

 the 3g/4g sutures corresponding to the similar struc- 

 tures in the plates of the epitheca and hypotheca adja- 

 cent to the girdle (fig. 16A). 



Lists. Cingular lists 8 to 10 microns wide, usually 

 somewhat broader at the left ventral side. They are 

 strengthened at the base by short ridges which connect 

 with similar strengthenings in the girdle and body plates, 

 and at the outer edge by ridges which extend about half- 

 way to the center of the list (pi. 1, fig. A). 



The apical list (fig. 15; pi. 1, fig. C) encircles the 

 apex dorsally and laterally, but ventrally it runs down 

 each side of the ventral apical platelet and extends part 

 way down the sides of the first apical plate. It forms a 

 collar around the apex about 2 microns in height, slight- 

 ly funnel-shaped with serrated edge. Toward the poste- 

 roventral ends, the list diminishes in width gradually 

 and terminates on each side of the first apical plate a- 

 bout 5 microns below the anterior end of that plate. Lat- 

 erally, from the apex, there is a list on each side which 

 runs from the apical list posteriorly. The right lateral 

 list (r.1.1.) extends down the suture between the third 

 and fourth apical plates, and the left lateral list (1.1.1.) 

 extends down the suture between the second and third 

 apicals. These lists are of the same width as the apical 

 list near the apex, but narrow gradually and end at points 

 about two-thirds of the way down the third apical plate. 

 The origin of the apical and lateral lists is not as might 

 be expected from a study of an intact specimen. Separa- 

 tion of the apical units, as shown in figure 15B, demon- 

 strates the attachment of the lists. The apical list is 

 composed of three parts: the dorsal, right, and left seg- 

 ments. The dorsal segment of the apical list (d.l.) and 

 the lateral apical lists are continuous and are attached 

 to the third apical plate along its anterior and lateral 

 edges. The left segment of the apical list (l.a.l.) with 

 its continuation down the ventral side of the horn is at- 

 tached to the second apical plate along its anterior and 

 anteroventral edges. The right segment of the apical list 

 (r.a.l.) with its continuation down the ventral side of the 

 horn is attached to the fourth apical plate along its an- 

 terior and anteroventral edges. 



In an intact specimen it is difficult to see the apical 

 list except where one looks through considerable thick- 

 ness of the list in the arcs which are tangent to the lines 

 of vision. It thus appears as two spines in whatever 

 longitudinal view the specimen is observed. This ex- 

 plains the real nature of the prominences which are fre- 

 quently shown in figures of this species. 



The more carefully drawn figures of this species 

 have shown that the first apical plate terminates below 

 the apex, but the space anterior to it has been indicated 

 as a separation continuous with the apical pore. This 

 space, as is now shown, is occupied by the ventral api- 

 cal platelet. 



The ventral area is almost entirely bounded by a 

 system of lists which are attached to the body plates 



