30 



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



The distribution of P. crassipes at the Carnegie 

 stations is unexplainable unless we assume that the oc- 

 currence is sporadic. The species is eurythermal and 

 has been found by other investigators over wide areas of 

 the Atlantic, from the tropics to the North Sea, yet it was 

 absent from all Carnegie stations in the Atlantic. In the 

 Pacific, in the Carnegie collection, it occurred mostly 

 in the warmer regions but was not found on the line of 

 stations between Guam and San Francisco, which passes 

 through both cold- and warm- water regions. It did oc- 

 cur, however, in the southeastern Pacific in water of 

 low temperature. 



Peridinium truncatum n.sp. 

 (Figures 39-41) 



Dimensions . Total length (1^) including the apical 

 and right antapical horns 230 (196-265) microns. Length 

 (h) from tip of apical to posterior end of sulcal 161 (138- 

 210) microns. Diameter (d) 159 (130-200) microns. Di- 

 ameter (g) 80 (65-105) microns. Width of girdle 5-6 mi- 

 crons. Twelve specimens were measured. 



Shape . Body shape characterized by pronounced 

 features. Mid-body compressed anteroposteriorly in 

 girdle region, especially at the sides, to such a degree 

 that the lateral sides are scarcely thicker than the gir- 

 dle is wide (fig. 39E). In contrast with this extreme 

 compression of the mid-body, all the horns are so long 

 that the epitheca as well as the hypotheca is about 1.5 

 times longer than the dorsoventral diameter. The 1/d 

 ratio is 1.45 (1.27-1.57). The h/d ratio is 1.01 (0.91- 

 1.17). The angle a is 47° (34°-65°). The rapid taper of 

 the mid-body into the horns is roughly the same on all 

 sides; thus the horns are almost symmetrical and the 

 marginal curvatures of the mid-body are similar in ven- 

 tral and lateral views. All horns are tubular distally 

 and elongated. Distal one-third to one-half of antapical 

 horns are of uniform diameter. End of apical horn a- 

 bout 1.5 girdle widths wide; ends of antapicals about 1 

 girdle width. Antapical horns truncate to slightly round- 

 ed, but never pointed. 



In apical view the body is basically reniform but 

 with ventrolateral limbs greatly elongated. Since the 

 prolongation of these limbs is a fundamental character- 

 istic of the species, it is necessary to express it numer- 

 ically in order that a proper delimitation of the species 

 may be obtained. For this reason, the width of the limb 

 (r) was measured as described on page 13 (fig. 8). This 

 width is best expressed as a fraction of g. The r/g ratio 

 in this species is 0.47 (0.43-0.51). Although this ratio 

 is sufficient to characterize the species, it is notewor- 

 thy that the length of each limb, measured from the apex 

 of the body, is at least 1.25 times the dorsoventral di- 

 ameter of the body. Another measurable feature which 

 is significant in this connection is the ventral projection 

 of these limbs; this projection, beyond the median ven- 

 tral point, is usually 0.5 times the dorsoventral diame- 

 ter of the body. The g/d ratio is also characteristic, 

 being 0.48 (0.41-0.54). 



There is practically no real displacement of the 

 girdle, although there is frequently an apparent one if the 

 specimen is not seen in an exact ventral view. This ap- 

 parent displacement may be dextral or sinistral depend- 

 ing on the direction of displacement of the specimen. 

 Girdle ends separated by the wide anterior portion of 

 the ventral area. In lateral view the girdle is seen to be 



inclined about 15° to the longitudinal axis. It is only 

 slightly concave. 



Ventral area occupies a large part of ventral sur- 

 face of body. About half the area of the right ventral 

 portion of the hypotheca and part of the posterior left 

 portion are composed of elements of the ventral area. 

 These parts of the ventral area occupy an integral part 

 of the body and are set into the general body contours 

 without any break. Sulcus proper composed of the cen- 

 tral elements of the ventral area (see below). Flagellar 

 pore oval, about 3 girdle widths long, and situated some- 

 what anterior to center of ventral area. The sulcus is 

 so rotated on its longitudinal axis that the pore faces to 

 the left rather than ventrally. Pore covered by sulcal 

 lists so that it does not face the exterior directly. 



Plate pattern . Pattern of major body plates typical 

 of divergens group. At the apex of body, there are 2 

 platelets in addition to the apical plates. Ventral apical 

 platelet long and narrow, more than 7 girdle widths long, 

 lying just anterior to the first apical and between the 

 second and fourth (fig. 39C, E). Apical pore platelet is 

 a ring platelet, set inside the ends of apicals 2, 3, and 

 4, and the ventral apical platelet. 



Epithecal pattern not symmetrical In respect to pre- 

 cingular plates. Precingular 2 is much narrower, both 

 relatively and actually, than corresponding plate on right 

 side, precingular 6. The girdle border of pr2 usually 

 less than one-fourth that of pr6 (fig. 39A). To compen- 

 sate for this, pr3 has a much longer girdle margin than 

 its fellow pr5 on the right side. Precingular 2 narrows 

 very markedly toward girdle, so that its width at girdle 

 may be as little as one-half its greatest width. 



Girdle composed of four unequal plates (fig. 39G). 

 First girdle plate somewhat narrower than long (fig. 

 39D). Second and fourth extend on either side of sulcus 

 almost to the most ventrolateral points of girdle on each 

 side. The rest of the lateral and dorsal parts of girdle 

 consist of the extremely long third plate. 



Pattern of hypothecal plates typical of divergens 

 group. Sulcal plates take a prominent place in the ven- 

 tral aspect of body, a characteristic feature in this spe- 

 cies. Right sulcal plate occupies a large part of ventral 

 surface of hypotheca. The left limb of the posterior sul- 

 cal plate fits into the major body plates between the first 

 postcingular and first antapical (fig. 39E) in such a way 

 that it might be mistaken for a major body plate in un- 

 dissected specimens. 



Ventral area composed of five plates, all external 

 (fig. 39D). Anterior sulcal plate occupies most of the 

 area between the ends of the girdle and forms anterior 

 margin of flagellar pore. It bears a process at its pos- 

 terior end which projects into the cell body. Left sulcal 

 plate occupies the left central and the posterior central 

 regions of the ventral area and, in its anterior half, 

 forms the left margin of the flagellar pore. Posterior 

 sulcal plate more or less U-shaped, forms posterior 

 part of ventral area, is far removed from flagellar pore, 

 and is very complex. Its left limb forms a large, four- 

 sided area in the hypotheca, extends from antapex half- 

 way to girdle, and fits into the hypothecal pattern poste- 

 rior to first postcingular. Its right limb lies posterior 

 to sulcal plate and is also raised to a position where it 

 must be considered part of the hypothecal pattern (fig. 

 39E). The right sulcal plate , which is the largest in the 

 ventral area, extends from posterior sulcal plate to epi- 

 theca, where it has a narrow extension lying against the 

 distal end of girdle and against seventh precingular. Ii. 



