FAMILY PERIDINIACEAE 



23 



Perldinlum depressum var. convexius n.var. 

 (Figure 26) 



Dimensions . Length of body (i) 122 (105-135) mi- 

 crons. Diameter of body (d) 81 (76-88) microns. Width 

 of girdle about 5 microns. Three specimens were meas- 

 ured. 



Shape . Body very convex in ventral view and does 

 not flare out in girdle region as in P. depressum . The 

 h/d ratio is 1.08 (0.96-1.21). The angle a is 128° (114°- 

 139°). In the girdle region the body is compressed dor- 

 soventrally so that it is not so convex in lateral view. 

 The g/d ratio is about 0.70. Antapical horns similar to 

 those in P. depressum although somewhat more abrupt- 

 ly pointed. The a/d ratio is 0.40 (0.36-0.46). The b/a 

 raUo is 0.19 (0.16-0.23). 



Plate pattern . Epithecal tabulation is of the first 

 symmetrical pattern. Other parts of theca as in main 

 species. 



Comparisons. This variety exhibits a peculiar com- 

 bination of features. It shows a tendency toward a 

 rounding of the lateral contours of the body as in var. 

 parallelum . but retains the horn length of P. depressum . 

 There is also a tendency toward a dorsoventral flatten- 

 ing, showing a convergence toward the shape of P. de- 

 pressum var. rectius and P. oceanicum var. tenellum . 

 Variety convexius can be distinguished from P. depres - 

 sum by its large angle oc; from var. parallelum by its 

 longer antapical horns; and from var. rectius by its con- 

 vex lateral contours. 



Distribution . This variety was found at 11 stations: 

 3 in the Atlantic and 8 in the Pacific. There are 28 rec- 

 ords of occurrence: 10 rare, 17 occasional, and 1 com- 

 mon. The records were less frequent for the surface; 

 there were 5 records for the surface, 11 for 50 meters, 

 and 12 for 100 meters. There are 18 net records and 

 10 pump records. The variety was found in both hemi- 

 spheres, from May to August in the northern, and from 

 November to January in the southern. 



In the Atlantic tliis variety was found at three sta- 

 tions, all in or along the Gulf Stream, between longi- 

 tudes 46° and 54° west (fig. 20). In the Pacific it oc- 

 curred in three regions: (1) off Colombia, station 39; 

 (2) in the Humboldt Current, off Peru, stations 69 and 

 70; (3) in a series of stations in the region of Japan, sta- 

 tions 109-112, 116. 



This form probably has its origin in regions of 

 warm water and it does not seem to endure transfer into 

 cold conditions. The surface temperatures at the sta- 

 tions where it occurred at any depth varied from 16 !l 

 to 27°.4 C. The ranges of hydrographlc conditions insi- 

 tu were as follows: temperature, 6°.7 to 27°.4 C; salin- 

 ity, 33.8 to 36.5 o/oo; pH, 7.68 to 8.23; phosphate, 3 to 

 233 mg P04/m3. 



It may be concluded that this is a rare warm-water 

 form, sometimes being carried into colder regions. It 

 Is not common enough to be of value as an indicator of 

 the intrusions of water of tropical origin. Type locality: 

 Carnegie station 14. 



Peridinium depressum forma bisintercalares n.f. 

 (Figure 27) 



Dimensions . Length of body (i) 182 (160-205) mi- 

 crons. Diameter (d) 132 (125-140) microns. Width of 

 girdle about 5 microns. Two specimens were measured. 



Shape . Body as in P. depressum . The h/d ratio is 

 0.98 (0.95-1.02). The angle a is 96° (95°-96°). Length 

 and divergence of antapical horns variable. The a/d 

 ratio is 0.37 (0.32-0.43). The b/a ratio is 0.12 (0.10- 

 0.15). 



Plate pattern . Epithecal tabulation is of the first 

 symmetrical pattern except that there are only two in- 

 stead of three anterior intercalary plates. In the meas- 

 ured specimens the x- ratio was +0.46 and +2.50. 



Comparisons . The distinctive feature of this form 

 is the presence of two, instead of three, anterior inter- 

 calary plates, a peculiarity also occurring in P. oceani - 

 cum (p.24). This pattern is not caused simply by the 

 fusion of two of the intercalary plates. Although one 

 specimen of P. oceanicum was found that indicated such 

 a fusion (p.25), in all other cases some more fundamen- 

 tal rearrangement had occurred. The suture between 

 the two plates was located medially where no suture 

 normally occurs when three plates are present, and the 

 dorsal pattern was quite symmetrical. 



Remarks . Since the subgenus Peridinium is founded 

 on the presence of three anterior intercalary plates, the 

 discovery of the two-intercalary forms in P. depressum 

 and P. oceanicum necessitates the revision of the de- 

 scription and fundamental classification of the genus. 

 This plate pattern was too common in the material stud- 

 ied to be passed over simply as "aberrant." It was 

 estimated that from 1 to 2 per cent of the specimens of 

 P. depressum and P. oceanicum in our material showed 

 this pattern, although in the former species it occurred 

 only at one station and in the latter at three. 



Distribution . This form was found only at station 

 13 at 50 meters on the Grand Banks of Newfoundland in 

 August 1928. The hydrographlc conditions were as fol- 

 lows: temperature, -1°6 C; salinity, 33.4 o/oo; pH, 7.87; 

 phosphate, 59 mg P04/m3. Type locality: Carnegie 

 station 13. 



Peridinium depressum forma multitabulatum n.f. 

 (Figure 28) 



Dimensions . Length of body Q) 160 microns. Width 

 of body (d) 135 microns. Width of girdle about 5 microns. 

 One specimen measured. 



Shape . Body shape as in P. depressum . The h/d 

 ratio is 0.90. The a/d ratio is 0.28. The b/a ratio is 

 0.24. The angle a is 99°. 



Plate pattern. The first apical plate is divided me- 

 dially by a suture without intercalary striae; the first 

 precingular is divided transversely; the fourth precin- 

 gular is divided by a suture in the longitudinal axis; the 

 first and second intercalary plates are displaced to the 

 left so that 2a touches 2pr and the suture between 2a 

 and 3a lies in the longitudinal axis of the body. There 

 seems to be a tendency toward formation of a cleavage 

 plane, cutting the body into right and left valves as in 

 the Dinophysoidae. 



Comparisons . This form, of which only one speci- 

 men was recorded, is peculiar only in its tabulation. 



Remarks . The aberrant behavior of the tabulation 

 in this form strikingly demonstrates the caution which 

 must be applied in using the plate pattern as the funda- 

 mental feature of classification. 



Distribution . This form occurred in the surface 

 sample at station 2 in the North Atlantic Drift in May 

 1928. The hydrographlc conditions were as follows: 



