FAMILY PERIDINIACEAE 



33 



1.07 (0.95-1.28). Epitheca almost triangular, with apex 

 more or less prolonged into apical horn, which may be 

 almost absent or as much as 2 girdle widths long. Hy- 

 potheca shorter than epitheca. Thus, the girdle is some- 

 what posterior. Hypotheca subtruncate with indentation 

 at sulcus. The two antapical processes thus formed can 

 scarcely be termed horns. Body well rounded in girdle 

 region. In apical or antapical view ovate, compressed 

 dorsoventrally. The s/d ratio is 0.68 (0.53-0.90). In 

 side view body also subovate. Girdle dextral, displaced 

 0.5 to 1.0 girdle width, not excavated, inclined (in side 

 view) at an angle of about 25°. No girdle overhang. 

 Ventral area angular in outline, extending from anterior 

 margin of girdle to antapex, and deeply embedded in 

 body. Sulcus lies to the left, overhung by the major body 

 plates. Flagellar pore oval in outline, centrally located 

 in ventral area and covered by the right accessory and 

 left sulcal lists (fig. 43B, C). 



Plate pattern . Epithecal tabulation para, hexa. A- 

 pex has a ring platelet and a ventral apical platelet (fig. 

 43A, D). Four girdle plates. The first girdle plate is 

 wider than long and extends somewhat into sulcus (fig. 

 42H). Second girdle plate short and squarish. The third 

 girdle plate comprises most of the girdle and encircles 

 the body almost to ventral area on right side (fig. 42F). 

 Fourth girdle plate short and rectangular, about twice 

 as long as wide, located at distal end of girdle. 



Ventral area composed of six plates: anterior, left, 

 posterior, posterior accessory, right, and right acces- 

 sory (fig. 42H). Anterior sulcal plate (as) long, extend- 

 ing from apical plate to anterior edge of flagellar pore. 

 At the pore it has an internal spongy structure com- 

 posed of prongs or forks which fit into similar struc- 

 tures on the left and right sulcal plates making an inter- 

 locked joint. Left sulcal plate (Is) forms left edge of 

 flagellar pore and its anterior end touches first girdle 

 plate. It has a thickened ridge along its right edge which 

 forms left edge of pore. Posterior sulcal plate (ps) 

 forms posterior part of ventral area and is almost U- 

 shaped. Its left arm very narrow and carries posterior 

 segment of left sulcal list. Its right arm lies posterior 

 to right sulcal plate (rs). The latter plate occupies 

 most of right side of ventral area and posterior half of 

 its left edge forms the right edge of flagellar pore. It 

 bears the thick right accessory sulcal list, which pro- 

 jects over the pore, as well as internal processes sim- 

 ilar to those in P. depressum . There is, however, no 

 detachable internal plate. Between the right and the 

 posterior sulcal plates, at the posterior edge of pore, 

 there is a very small irregular plate, the posterior ac - 

 cessor y sulcal plate. It is not more than two to three 

 microns long and thus of almost indeterminable shape. 

 Anterior to the right plate and opposite distal end of gir- 

 dle, there is a short rectangular plate, the right acces - 

 sor y sulcal plate . This plate has been found only in P. 

 pallidum , which was the only one investigated with a 

 right-handed girdle. It is possible that the presence of 

 this plate goes with the right-handed girdle, for in such 

 a displaced girdle either the right sulcal plate must be 

 longer or else there must be an additional plate to oc- 

 cupy the increased distance on the right side of the sul- 

 cus between the distal girdle end and the antapex. 



Body wall . Surface of body plates covered with ir- 

 regular short ridges and tubercles. Pores scattered 

 over the major plates and the right, left, and posterior 

 sxilcal plates. Very irregular row of extremely small 

 pores in mid-line of girdle plates. Very small trans- 



verse ridges may exiend from outer margins of girdle 

 plates toward middle. Intercalary zones common and 

 may be quite wide. Intercalary striae run across these. 

 Rabbeting membranes well developed and may be as 

 much as 2 girdle widths wide. 



Lists. Girdle lists well developed, usually about 1 

 girdle width wide. Apical lists well developed, some- 

 times more than 1 girdle width wide, similar to those in 

 P. depressum but wider and more extensive (fig. 43A, 

 D). Apical list encircles apex; composed of 3 segments: 

 dorsal, right, and left. Right and left segments (r.a.l., 

 l.a.l.) are attached to apical and ventral edges of second 

 and fourth apical plates and continue down ventral side 

 of body sometimes as far as girdle. In this case the 

 proximal segments of these lists are attached to second 

 and sixth precingular plates. Dorsal segment of the ap- 

 ical list (d.l.) attached to third apical plate and continu- 

 ous with lateral lists. Right lateral list (r.1.1.) and left 

 lateral list (1.1.1.) run down sides of body, frequently to 

 girdle, in which case their intermediate segments are 

 attached to first and third intercalary plates and the 

 proximal, or girdle, segments are attached to third and 

 fifth precingular plates (fig. 42B). 



Sulcal lists are also well developed. Left sulcal list 

 extends laterally over sulcus, about 1 girdle width wide. 

 Its anterior segment attached to first postcingular and 

 continuous with posterior girdle list. Its posterior seg- 

 ment attached to left arm of posterior sulcal plate. Left 

 sulcal list terminates posteriorly in mid-region of this 

 plate (fig. 42H). Posterior sulcal list, which is attached 

 to sulcal margins of antapical plates, and the right sul- 

 cal list, which is attached to the second antapical and 

 fifth postcingular plates, are poorly developed except 

 where they converge and run up the right antapical spine. 

 (Fig. 42H). When intercalary striae occur alongthe right 

 border of the ventral area there is a weak development 

 of the right sulcal list on each edge of the zone. Right 

 accessor y sulcal list, which is attached to the right sul- 

 cal plate, is well developed and overhangs flagellar pore. 



Other body lists may develop to a minor degree a- 

 long the sutures of the major body plates. 



Spines . The two rounded processes of the antapex 

 are terminated by spines; the right usually longer than 

 the left. Length of right spine is 12.4 (7-17) microns. 

 Each spine composed of united short body lists. On the 

 right spine, one of these lists is continuous with left 

 sulcal list and one with posterior sulcal list. There are 

 at least two others which run out of the second antapical 

 plate with no relation to sutures. The left spine has four 

 or more short lists which bear no relation to sutures. 

 One of these articulates transversely with, and runs out 

 of, the left sulcal list. It can be disarticulated from it. 

 (Fig. 42H). 



Variations . This species varies considerably in 

 size and shape. The length varies by three times (44- 

 128 microns). The compression of the body is rather 

 uniform in most specimens (g/d ratio is about 0.68) but 

 some specimens were found with more circular girdle 

 section, with g/d ratio as high as 0.90 (g/d ratio of clr- . 

 cle, of course, 1.00). Width of girdle is remarkably 

 constant in its absolute values, about 5 microns. This 

 is particularly noticeable in small specimens, where the 

 girdle is, then, relatively wide. The plate pattern in this 

 species seems to be in a fairly stable state. Only one 

 aberration was found. In this the fourth and fifth pre- 

 cingular plates were fused. The length of the antapical 

 spines varies in accordance with the variation in devel- 



