28 



STUDIES IN THE MORPHOLOGY, TAXONOMY, AND ECOLOGY OF THE PERIDINIALES 



Twenty-two specimens were measured. 



Shape . Body short and wide, resembling somewhat 

 P. depressum var. parallelum Broch. TheJ/d ratio is 

 0.95 (0.80-1.04). The h/d ratio is 0.76 (0.60-0.86). In 

 ventral view both the epitheca and the hypotheca rather 

 abruptly constricted distally. The angle a is 89° (74°- 

 102°). Apical horn 1/4 to 1/3 total length of epitheca; 

 base broad. Apex about 1 girdle width in diameter. Ant- 

 apical horns terminated by solid spines. Right horn a- 

 bout 2 girdle widths longer than the left. There is no 

 dorsoventral compression of the body in the girdle re- 

 gion, so that in apical view, the body is almost circular 

 except for the deep indentation at the sulcus, which is 

 about 3 girdle widths deep. The g/d ratio is 0.73 (0.64- 

 0.80). There is sometimes a slight development of ven- 

 tral limbs in the girdle region (as seen in apical view), 

 but this is not very marked. 



The girdle is equatorial, sinistral, displaced about 

 2 girdle widths. The ends of the girdles themselves 

 are scarcely (about 0.5 girdle width) displaced, however, 

 owing to the posterior curvature of the distal end. There 

 is no girdle overhang; the girdle ends are separated by 

 the anterior end of the ventral area. Girdle slightly con- 

 cave. In side view the girdle is seen to be inclined at an 

 angle of 75° to the longitudinal axis (the axis running 

 from the apex to the antapex of the body). 



The ventral area extends from the anterior edge of 

 the girdle posteriorly to the antapex, which is located 

 almost centrally (when specimen is seen in antapical 

 view). Area subtruncate anteriorly; rounded posterior- 

 ly; left side almost straight; right side angular so that 

 the greatest width, equaling about 4 girdle widths, oc- 

 curs about one-third of distance from girdle to right 

 antapical horn. Right side of ventral area on a level 

 with, and taking a prominent place in, the ventral aspect 

 of main body; but left side of area scarcely evident on 

 the main body. Flagellar pore roughly elliptical in out- 

 line and about 2.5 girdle widths long, lying in the middle 

 of sulcus proper, which occupies left half of ventral a- 

 rea. Sulcus rotated on its longitudinal axis in such a way 

 that pore faces more to the left than ventrally. Sulcus 

 and pore covered by left sulcal list. 



Plate pattern . Epithecal plate pattern typical of di- 

 vergens group, with first apical touching precingulars 1, 

 2, and 7, and apicals 2 and 4; and with second anterior 

 intercalary touching only fourth precingular (fig. 37A). 

 Preclngular plates fairly evenly spaced around girdle. 

 Precingular 2 has a somewhat smaller girdle margin 

 than its fellow, precingular 6, on the right side, but this 

 discrepancy is not nearly so pronounced as in the case 

 of P>. truncatum (p. 28). 



Hypothecal pattern typical of genus (fig. 37C). At 

 apex there are 2 platelets in addition to apical plates, as 

 in P. depressum . Ventral apical platelet about 1 girdle 

 width long. Apical pore platelet a ring structure simi- 

 lar to that in P. depressum . 



Girdle consists of four unequal plates (fig. 37D). 

 First girdle plate (gl) is a short, squarish plate adjacent 

 to the ventral area. The second and fourth (g2, g4) about 

 equal In length and occupying the ventral portions on the 

 two sides of girdle. Lateral and dorsal sides of girdle 

 composed of the long third girdle plate (g3). 



Ventral area composed of six plates (fig. 37E, F). 

 Only four of these are visible in undissected specimens, 

 the other two hidden as in the case of P. depressum. All 

 plates homologous with those in P. depressum . a species 

 with a ventral area in many respects similar to that of 



P. crassipes . (See comparisons.) Anterior sulcal p late 

 (as) occupies most of the anterior part of the area and 

 forms anterior margin of flagellar pore. Left sulcal 

 plate (Is) comprises most of the left side of sulcus and 

 forms left edge of flagellar pore. Right sulcal p late (rs) 

 occupies most of right side of ventral area, is raised 

 practically to the level of the main body plates, as in P. 

 truncatum (p. 28), and forms right margin of flagellar 

 pore and sulcus proper. Posterior sulcal plate (ps) 

 forms posterior part of the area and is U-shaped in out- 

 line. Its right side is expanded into a squarish process 

 which lies posterior to right sulcal plate. Its middle 

 portion is very narrow. Its left side is elongated into a 

 slender process which borders posterior left edge of the 

 sulcus and is raised to the level of the body plates, fit- 

 ting in between the first postcingular and first apical 

 plates. This portion bears the posterior part of the left 

 accessory sulcal list on its inner margin. At the poste- 

 rior edge of flagellar pore between the right and the left 

 sulcal plates is a small, squarish, trough- shaped plate, 

 the posterior accessor y sulcal plate (pas). Along the 

 right side of pore, underneath the left edge of the right 

 sulcal plate, is a thin plate, the right internal sulcal 

 plate (ris), which has a posterior arm turned into the 

 body of the specimen. A similar projection into the pro- 

 toplast at the anterior edge of the pore is not a separate 

 plate but a process on the under side of the anterior sul- 

 cal plate. 



Body wall . Body plates and some sulcal plates cov- 

 ered with a light reticulation which is usually irregular- 

 ly raised at the angles of the meshes. Size of meshes, 

 except over rabbeting membranes, from 0.3 to O.Sgirdle 

 width. Over these membranes, in individuals without in- 

 tercalary zones, the meshes are noticeably larger, 0.6 

 girdle width or more; but reticulations are much less 

 well developed, the thecal wall appearing thinner; and no 

 pores occur. In the more finely reticulated areas, there 

 are fairly regularly spaced pores, usually one per mesh. 

 In these areas the thecal wall is thicker and the meshes 

 of the reticulations sometimes give the appearance of 

 pits, although their edges are always angular. The gir- 

 dle plates do not bear these reticulations, but have ir- 

 regular transverse ridges which usually extend only half - 

 way from the edge of the girdle, and may be connected, 

 forming a sort of reticulation. There are no pores in the 

 girdle although the median line of the girdle has a spon- 

 gy nature. Intercalary zones are common. 



Lists. Girdle lists about 1 girdle width wide and 

 strengthened by small ribs which extend centripetally 

 from outer margin halfway to body. These ribs are 

 placed from 0.2 to 0.3 girdle width apart. In the apical 

 region there is a system of lists similar to that in P. de- 

 pressum except that the most distal parts are less well 

 developed. The lists running down the body from the a- 

 pex are larger than in P. depressum , sometimes being 

 as much as 1.5 girdle widths wide. The lateral ones (at- 

 tached to ap3) usually visible in ventral and dorsal viewa 

 The right and left segments of the apical list (ral, lal, 

 fig. 37G), which run down the ventral side of the body 

 and are attached to the fourth and second apicals, re- 

 spectively, are clearly seen when specimen is viewed in 

 lateral aspect. 



Lists in the sulcal region well developed. Left sul - 

 cal list continuous with posterior girdle list and at- 

 tached to po; continuous posteriorly with the left acces - 

 sor y sulcal list (la), which it? attached to the right edge 

 of the left limb of the posterior sulcal plate (ps). The 



