20 



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



surrounding the area. On the left side a wide list, the 

 left sulcal list (1.1.), runs along the first postcingular 

 plate (to which it is attached) to the suture between this 

 plate and the first antapical plate. This list is a contin- 

 uation of the posterior cingular list. The posterior sul- 

 cal list (p.l.) extends from the above suture around the 

 posterior end of the sulcus to the suture between the 

 second antapical and the fifth postcingular plates. It is 

 attached to the sulcal edges of the two antapical plates. 

 Where this list turns from the sides of the sulcus down 

 to the posterior end, it runs parallel with the line of 

 vision when the specimen is viewed ventrally. When 

 viewed on an intact specimen, this list is visible fre- 

 quently only at the posteroventral corners of the sulcus, 

 where considerable thickness of the upturned list results 

 in greater interference of light and gives the impression 

 of two spines. Specimens have often been figured with 

 these two "spines" at the bases of the antapical horns. 

 The posterior right sulcal list (p.r.l.) extends from the 

 posterior cingular list to the suture between the fifth 

 postcingular and second antapical plates. It is attached 

 to the former plate and is continuous with the posterior 

 cingular list. The right and left sulcal lists, when seen 

 from the apical or antapical ends, appear as two spines 

 near the ends of the cingular lists and have frequently 

 been so drawn or referred to as "teeth" at the base of 

 the antapical horns. The anterior right sulcal list 

 (a.r.l.) is a structure of the epitheca. It borders the 

 right edge of the sulcus which extends into the epitheca, 

 but is attached to the sulcal edge of the seventh precin- 

 gular plate and is continuous with the anterior cingular 

 list. 



Variation . There is variation in the habitus of the 

 species, as can be seen from the specimens presented 

 in figvire 19. It should be noted that the differences in 

 the habitus of these specimens are due chiefly to the 

 variations in the length, thickness, and degree of diver- 

 gence of the antapical horns, and in the length of the 

 apical horn. That the pronounced differences in the 

 shape of the mid-body which are suggested by this figure 

 are more apparent than real, was demonstrated by pro- 

 portional measurements of the specimens. These meas- 

 urements all fell within the limits of the species (p. 16). 



For variation in the plate pattern, see p. 14. The 

 tabulation of the ventral area and girdle is constant. 



Comparisons . The species of the "Formenkreis" 

 P. depressum comprise the greater part of the section 

 Oceanica of Orthoperidinium . They can be distinguished 

 from the other species of the section by the inclined gir- 

 dle. Also, the other species are mostly much smaller, 

 e.g., P. obtusum Karsten and P. bulla Meunier. 



Within the "Formenkreis," P. depressum can be 

 distinguished from the other species by its body shape. 

 It Is very similar to P. oceanicum (see p. 15), but can be 

 distinguished from the latter by its lower _l/d ratio 

 (less than 1.20) and lower angle a (less than 105°). 



Perldinium claudlcanoides n.sp. of this "Formen- 

 kreis" can be separated from P. depressum by its cune- 

 ate antapical horns and by its b/a ratio, which is great- 

 er than 0.23. 



Historical . Bailey (1855) described and figured P. 

 depressum in an easily identifiable manner. Some early 

 authors, e.g., Pouchet (1883) and Cleve (1900b), consid- 

 ered this form to be identical with P. diver gens Ehren- 

 berg (1840). Others, e.g., Bergh (1881) and Meunier 

 (1910), were somewhat more discriminating and consid- 

 ered P. depressum to be identical only with the variety 



which Ehrenberg (1854) described as P. divergens y 

 reniform e. It is possible that this variety and P. de- 

 pressum are identical, but since the figure of Ehrenberg 

 (p. 240) cannot be identified with any measure of cer- 

 tainty, and since Bailey's figures are readily recogniza- 

 ble, it is advisable to retain the now universally accept- 

 ed name depressum for this species. 



Since the importance of the plate pattern in the clas- 

 sification of the species of this genus was not realized 

 until the present century, and since neither Ehrenberg 

 nor Bailey indicated the tabulation for the two species in 

 question, considerable confusion developed regarding 

 these forms. There was even a tendency to lump all 

 large forms of Peridinium under one species, viz., P. 

 divergens Ehrenberg, which was the first to be described 

 in the genus. 



After Broch (1910) and JOrgensen (1913) had demon- 

 strated the importance of tabulation in this genus, it be- 

 came necessary to determine the plate patterns of the 

 various species which had been described earlier. 



In the case of P. depressum this was not difficult, 

 as the figures of Bailey are easily identifiable. This 

 form has the Orthoperidinium tabulation, one which was 

 first indicated for this species by Pouchet (1883, pi. 21, 

 fig. 26) under the name of P. divergens var. reniforme . 



In the case of P. divergens . it is impossible to es- 

 tablish the tabulation except on an arbitrary basis, since 

 neither the description nor the figures of Ehrenberg are 

 identifiable. The first figures of tabulation shown imder 

 the name of P. divergens are those of Stein (1883), but 

 unfortunately, as Lebour (1925) states. Stein figured at 

 least three species under this name. It is not difficult, 

 however, to select figures 2 to 6 of plate 11, as those 

 which are most likely to represent Ehrenberg's P. di- 

 vergens. For the sake of clarifying the confused synon- 

 ymy of this species, these figures should be accepted, 

 as Lebour suggested, as establishing the tabulation of P. 

 divergens . The pattern is that of the section Divergens . 

 Metaperidinium , of Jorgensen. 



The specific distinctness of P. depressum and P. 

 divergens has been fully realized by most workers in 

 this field for a considerable period. Meunier, as late 

 as 1919, figured P. depressum under the name of P. di- 

 vergens ; however his argument to discard the former 

 name seems invalid. 



Peridinium kofoidii , described by Faure-Fremiet 

 (1908), is undoubtedly a long-horned form of P. depres - 

 sum with roughened antapicals. The apical view shows 

 only three apical plates, but this is probably an error in 

 drawing, for the dorsal apical plate is difficult to dem- 

 onstrate in this view. 



Distribution . Very common in the boreal Atlantic 

 Ocean (Paulsen, 1908; Lebour, 1925) and in the Antarc- 

 tic (Karsten, 1905; Peters, 1928). Matzenauer (1933) 

 found it frequently in the Indian Ocean and Forti (1922) 

 in the Mediterranean. Bohm (1936) reported it from the 

 Pacific between Hong Kong and Shanghai, where its oc- 

 currence was sporadic. It is apparently a cosmopolitan 

 species having centers of abundance in the cooler re- 

 gions of both hemispheres. 



In the Carnegie collection this species was found at 

 35 stations: 22 in the Atlantic, 13 in the Pacific. There 

 are 72 records of occurrence: 34 rare, 28 occasional, 

 6 common, and 4 abundant. It was found about equally 

 at the three levels, with 20 records for the surface, 17 

 for 50 meters, and 20 for 100 meters. There are 48 net 

 records and 24 pump records. It was found only in the 



