FAMILY PERIDINIACEAE 



21 



northern hemisphere and in the months from May to Sep- 

 tember. 



The Carnegie records exhibit a very curious distri- 

 bution, being widely scattered in the Atlantic but limited 

 in the Pacific (fig. 20). In the Atlantic the species oc- 

 curred from north of the Faeroes Islands as far south as 

 latitude 13°4 north. Its center of abundance was the 

 Grand Banks of Newfoundland, where it occurred in enor- 

 mous numbers clogging the collecting nets, particularly 

 at 50 and 100 meters depth. In the Pacific it was re- 

 stricted to a line of stations off Japan and the western 

 Aleutian Islands, stations 111 to 122. 



This species was found in a great range of hydro- 

 graphic conditions. The surface temperatures at the^ 

 stations where it occurred at any depth varied from6°.9 

 to 27°6 C. The ranges of hydrographic conditions in situ 

 were as follows: temperature, -l.°6 to 26.°7 C; salinity, 

 32.7 to 37.2 o/oo; pH, 7.86 to 8.27; phosphate, 3 to 184 

 mg P04/m3. 



It is obvious from the above data that the factor lim- 

 iting the distribution of P. depressum has not yet been 

 identified. Considering the varied conditions under 

 which the species was found in the Atlantic, it is diffi- 

 cult to explain its peculiarly restricted distribution in 

 the Pacific unless its occurrence is normally sporadic, 

 as Bohm (1936) indicated was the case in the southwest- 

 ern Pacific. 



It is of interest to note that the species exhibited its 

 greatest variation in the region of the Grand Banks of 

 Newfoundland (station 13), where conditions were ex- 

 treme. In this region the surface temperature was 11°27, 

 whereas at 25 meters 0°.75 was recorded. At 50 meters 

 the temperature was only -1°.64 and at 100 meters 1°.10. 

 Curiously enough, the richest collection of P. depressum 

 made by the Carnegie was at the Grand Banks station at 

 50 meters. It cannot be concluded that low temperatures 

 are favorable to this species, because it is quite possi- 

 ble that the production of this population occurred at a 

 higher level. It at least indicates that P. depressum can 

 endure low temperatures. The species seems to be eu- 

 rythermal to a high degree, for it was again found in the 

 tropics at temperatures as high as 26°.7. South of sta- 

 tion 15 (latitude 39° north), however, it was always 

 found below the surface, a fact which may have been due 

 to the higher temperatures prevailing at the surface. It 

 should be noted that the higher concentration of nutri- 

 ents which occurs below the surface in the warmer wa- 

 ters may have been responsible for this distributional 

 peculiarity. 



Since the species has a distribution which cannot be 

 correlated with any known hydrographic condition, it is 

 at present of no value as an oceanographic indicator. 



Peridinium depressum var. parallelum Broch 

 (Figures 21, 22) 



Peridinium parallelum Broch, 1906, pp. 153-157, fig. 2. 



Paulsen, 1907, pp. 11, 15; 1908, pp. 39, 54, fig. 68. 



Broch, 1908, p. 5. Broch, 1910, p. 52. Mangin, 1913, 



p. 221. Paulsen, 1913, p. 279, pi. 47. Pavillard, 



1931, pp. 56, 114-116,121, 123, 159, 161, 163, 167, 169, 



175, 183, 185, 187, pi. 2, fig. 7A, B. 

 Peridinium divergens var. Schutt, 1895, pi. 13, fig. 



43 (23). Meunier, 1910, pi. 1 (bis), figs. 3, 4. 

 Perldlniimi antarcticum Schlmper in Karsten, 1905, pp. 



37, 38, 40, 43-51, 53-57, 59-61, 64-68, 131-132, pi. 



19, figs. 1-4. Broch, 1906, p. 153. Paulsen, 1931 



p. 55. 



Peridinium depressum subsp. parallelum Broch, 1906, 



p. 151. 

 Peridinium divergens antarcticum . Karsten, 1907, pp. 



225, 416. 



Dimensions . Length of body (i) 132 (100-160) mi- 

 crons. Diameter of body (d) 120 (94-138) microns. 

 Forty-nine specimens were measured. 



Shape. This variety is usually more rotund than 

 the main species, with short and slender antapical 

 horns, frequently solid part way or entirely to the base, 

 usually straight and parallel. The h/d ratio is 0.84 

 (0.73-0.96). The a/d ratio is 0.24 (0.16-0.29). The b/a 

 ratio is 0.15 (0.10-0.20). Theanglectis 110° (94°-129°). 



Plate pattern . Tabulation of major body plates as 

 in P. depressum with addition of second asymmetrical 

 pattern found in one specimen. It should be noted that 

 the first symmetrical and first asymmetrical patterns 

 occurred in about the same proportion as in P. depres - 

 sum (fig. 10). Tabulation of girdle and ventral area also 

 as in main species. 



Variation and comparisons . Broch (1906) separat- 

 ed this variety on the basis of solid antapical horns. 

 This is not a good character, however, as Peters (1928) 

 has demonstrated. In our material the solidity of the 

 horns was a function of the thickness of these struc- 

 tures. The thinner horns were solid part way or en- 

 tirely to the base. 



The angle a. is low owing to the greater convexity 

 and shortness of the body. When the specimens were 

 examined, this convex curvature of the body seemed 

 characteristic, and in the early part of these investiga- 

 tions it was .thought that the variety could be distin- 

 guished on the basis of this character alone. When 108 

 random specimens of the main species and of var. par- 

 allelum were measured, however, and the frequency of 

 their angle a values was plotted, it was found that this 

 feature is not specific (fig. 23). Rather than two sepa- 

 rate groups, the frequency distribution suggests a ser- 

 ies of variable and inter grading units. Since these re- 

 sults could not be corroborated by the study of addition- 

 al characters, the attempt to separate the variety on 

 this basis was abandoned. 



Because of the fact that var. parallelum appeared 

 to have shorter horns than P. depressum . a study was 

 made of the relative lengths of the antapical horns. The 

 a/d ratio, representing the length of the right antapical 

 horn in relation to the width at the girdle, was comput- 

 ed for 108 random specimens. The values obtained 

 were grouped into 0.03-unit classes and the frequencies 

 plotted (fig. 24). This study also showed that there is no 

 complete separation of two groups. There is an indica- 

 tion of two groups, however, Intergradlng at the a/d 

 class of 0.31. This grouping corresponds well to other 

 differences in habitus, etc. This was the best numeri- 

 cal measure that could be found to delimit var. paral - 

 lelum . a fact that clearly shows the uncertain status of 

 this unit. 



Since further knowledge of the differences in the 

 distribution of these two forms is needed, it is desira- 

 ble that the occurrence of the forms be kept separate in 

 plankton lists. As shown by the above frequency studies, 

 the most effective separation of the variety can be made 

 by means of the relative length of the antapical horns. 

 In the present work, the a/d ratio of 0.30 was taken as 

 the point of separation between P. depressum and var. 

 parallelum . 



