32 



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



of the right arm of the posterior sulcal plate. The su- 

 ture between pol and antl, although not exactly compar- 

 able in the two species, shows a fundamental difference 

 in its course. In P. depressum it reaches the sulcus 

 far posteriorly, whereas in P. truncatum it reaches the 

 sulcus about midway of its length, about at the midpoint 

 of the left sulcal plate. In P. trtincatum the left limb of 

 the posterior plate extends anteriorly to meet this su- 

 ture. All together these differences in sulcal plate pat- 

 tern are greater than the differences between the typi- 

 cal epithecal patterns of the ortho and meta groups of 

 the genus. 



The sulcal list systems of P. truncatum and P. de - 

 pressum are as fundamentally different as are the plate 

 patterns. In P. depressum the functional border list of 

 the right side of the ventral area is the right sulcal list 

 (rl, fig. 17), which is attached to the body plates along 

 the right edge of the ventral area. In P. truncatum this 

 list is practically imdeveloped and the fimctional right 

 border list is the right accessory sulcal list (ras, fig. 

 39), which in P. depressum is located deep within the 

 sulcus proper. There is a similar difference between 

 these two species as regards the left side of the sulcus. 

 The posterior girdle list and the anterior part of the 

 left sulca! list are continuous in both species, but the 

 posterior part of the left sulcal list has quite different 

 positions in the two species. In P. depressum this part 

 is located at the left margin of the deep sulcus and is 

 continuous with the anterior part of the left sulcal list 

 and with the posterior sulcal list, which, in turn, runs 

 around to meet the right sulcal list. Thus, the ventral 

 area is bordered by three lists: the primary left, the 

 posterior, and the right sulcal lists. In P. truncatum , 

 concomitant with the expansion of the ventral area, 

 these bordering lists have been subordinated or lost 

 compleiely and there has been a development of acces- 

 sory lists bordering the sulcus proper, which occupies 

 only a limited part of the ventral area. The anterior 

 part of the left sulcal list connects with the left acces- 

 sory sulcal list (la), which is joined to the posterior ac- 

 cessory sulcal list (pal); this, in turn, is joined to the 

 right accessory sulcal list (ras). 



The ventral area of P. truncatum is not so different 

 from that of P. crassipes as it is from that of P . depr es - 

 sum. For a comparison of these features in P. trunca - 

 tum and P. crassipes . see "Comparisons" under P. 

 crassipes . 



The very marked differences in the structure of the 

 ventral areas which were found in these species, as ex- 

 emplified by the comparison of P. depressum and P. 

 truncatum . are of the greatest importance to the taxon- 

 omy of the genus. It is probable that the future classi - 

 fication of the species of Peridinium will be based on 

 the morpholog y of the ventral area. 



Historical . The figures of (%amura (1912, pi. 4, fig. 

 58a-c) given under P. fatulipes probably refer to this 

 species. Otherwise, it apparently has not been reported 

 before. 



Distribution . Peridinium truncatum was found at 

 24 stations, all in the Pacific. There are 46 records of 

 occurrence: 27 rare, 16 occasional, and 3 common. It 

 was foimd less at 100 meters than at the other levels, 

 with 16 records for the surface, 20 for 50 meters, and 

 10 for 100 meters. There were 41 net records and 5 

 pump records. 



The species was not found south of 16° south nor 

 north of 34° north. There was no definite center of a- 



bundance. The records of "occasional" were scat- 

 tered throughout the range. One of the records of 

 "common" was near Panama (station 35a), the other 

 two were northeast of Samoa (station 157). 



The surface temperatures at the stations where the 

 species occurred at any depth varied from 18°.7 to 

 29°.4 C. The ranges of hydrographic conditions in situ 

 were as follows: temperature, 14°.4 to 29°.2 C; salinity, 

 29.7 to 35.9 o/oo; pH, 7.82 to 8.39; phosphate, 4 to 189 

 mg P04/m3. 



This is apparently a strictly tropical species, per- 

 haps confined to the Pacific. Waters of low nutrient 

 content are apparently no barrier to it, as there were 5 

 records in water containing less than 10 mg P04/m3. 

 Type locality: Carnegie station 35. 



Peridinium truncatum forma acutum n.f. 

 (Figures 40C, D) 



Dimensions . Length of body ( 1_) 238 microns. 

 Length (h) 168 microns. Diameter (d) 168 microns. Di- 

 ameter (g) 98 microns. Width of girdle 6 microns. One 

 specimen measured. 



Shape . Body shape close to average for species in 

 all respects except in the length of antapical horns and 

 theg/d ratio. TheJ/d ratio is 1.42. The h/d ratio is 

 1.00. The g/d ratio is 0.58. The r/g ratio is 0.46. The 

 angle a is 63°. Antapical horns more slender, with 

 pointed ends, the right more so than the left. The spec- 

 imen observed had broad intercalary zones, so that 

 there was a wider spread of the antapical horns than 

 otherwise would have been the case. Dorsoventral di- 

 ameter greater than in the main species. The angle a 

 is near the maximum for the species. In all other re- 

 spects the shape fits description of main species. Plate 

 pattern apparently similar in all details. 



Distribution . This form was found only at station 

 104, in the western Pacific northwest of the Marshall 

 Islands, in May. It was collected in the 100- meter tow. 

 The hydrographic conditions in situ were: temperature. 

 25°.3 C; salinity, 35.3 o/oo; pH, 8.21; phosphate, 7 mg 

 P04/m3. Type locality: Carnegie station 104. 



Peridinium pallidum Ostenfeld 

 (Figures 42, 43) 



Peridinium pallidum Ostenfeld, 1899, p. 60. Ostenfeld. 

 1900, p. 58. Ostenfeld, (1903) p. 581. figs. 130, 131. 

 Cleve, 1900a, p. 17, pi. 7, figs. 21, 22. Jorgensen, 

 1905, p. 110. Paulsen, ld07, p. 14. Paulsen, 1908, 

 pp. 48-49, fig. 60. Broch, 1910b, p. 45, fig. 17. 

 Meunier, 1919, pi. 15, figs. 24-29. Lebour, 1925, 

 p. 134, pi. 28, fig. la-d. Peters, 1928, pp. 31-33, 

 fig. 7a-c. 



Peridinium pellucidum Gran, 1902, p. 186. 



not Peridinium divergens pallidum Karsten 1906, pi. 23, 

 fig. 13a, b. 



Dimensions . Length of body, including right antap- 

 ical spine, (i) 94 (44-128) microns. Length (h) 78 (38- 

 107) microns. Transdiameter (d) 73 (35-98) microns. 

 Diameter (g) 45.9 (33-56) microns. Angle a 121°5 

 (105°-148°). Width of girdle 5 (3-6) microns. Eighteen 

 specimens were measured. 



Shape . Body in ventral view roughly squarish In 

 outline, somewhat longer than broad. The h/d ratio is 



