FAMILY PERIDINIACEAE 



25 



Plate pattern . The tabulation is of the first sym- 

 metrical pattern, rarely of the first asymmetrical. The 

 latter was observed in one specimen only. The x-ratlo 

 varied from + 0.70 to +1.30in the symmetrical patterns 

 and was -1.1 in the specimen with the asymmetrical pat- 

 tern. Tabulation in other regions as in P. depressum. 



Body wall . Details of thecal structure the same as 

 in P. depressum . In the tropical specimens the body 

 walls were thinner and the surface sculpturing less de- 

 veloped than in the northern representatives. 



Variation and comparisons . This species is close- 

 ly related to P. depressum , from which it may be dis- 

 tinguished by its greater h/d ratio (greater than 1.20) 

 and by its greater angle <x (greater than 105°). (See p. 

 15.) 



There are a great number of variants of P. oceani - 

 cum , which have caused considerable trouble among in- 

 vestigators of this group. (See below.) It is not likely 

 that this group will be dealt with in an orderly manner 

 until the description of the body shape is expressed nu- 

 merically, thus allowing frequency studies for the es- 

 tablishment of groupings and intergradations. 



From the Carnegie material 82 specimens with h/d 

 ratios greater than 1.20, i.e., specimens belonging to P. 

 oceanlcum and varieties, were selected at random from 

 widely scattered oceanic stations in the North Atlantic 

 and Pacific, and measured. The frequency of their 

 length classes (fig. 31) indicates two distinct groups: 

 one with lengths from 140 to 200 microns; the other with 

 lengths from 220 to 270 microns. Although the two 

 groups are distinct, they are, nevertheless, close togeth- 

 er. Since observation of the habitat corroborates this 

 grouping, it seems justifiable, for the present at least, 

 to consider the body length as a specific feature sepa- 

 rating P. oceanicum from its varieties. The group with 

 greater length is the main species. 



In P. oceanicum proper the body form shows a cer- 

 tain stability, but in the group of smaller forms the var- 

 iation is extreme (fig. 33). No secondary grouping of 

 these smaller forms could be accomplished. Therefore 

 they were lumped into a single group, var. tenellum n. 

 var. 



In addition to this 'variety, which was separated on 

 the basis of body length and form, three other variants 

 of P. oceanicum were found which were aberrant in oth- 

 er features; forma spiniferum n.f. and f. tricornutum 

 n.f. were segregated on the basis of accessory horns 

 (pp. 24 and 25), and f. bisintercalares n.f. was estab- 

 lished on the presence of only two anterior intercalary 

 plates (p. 24). 



Historical . Vanhoffen (1897a, pi. 2, fig. 5) figured 

 a specimen similar to P. depressum but longer and more 

 slender. No reference was made to this figure in the 

 text; but in another paper VanhOffen (1897b) gave a list 

 of species including the name "P. oceanicum Vanhof- 

 fen," which has generally been supposed to refer to this 

 figure. No description of the species was given by Van- 

 hoffen nor was the plate pattern shown. 



This form has been almost universally accepted as 

 a distinct species related to P. depressum (Jorgensen, 

 1905; Broch, 1906, 1910; Paulsen, 1907, 1908, 1913; 

 Mangin, 1913; Pavillard, 1931). Peters (1928) alone has 

 combined it with P. depressum . Our own observations 

 also indicate that it should be given specific rating. At 

 the same time, it is certainly not so easily distinguish- 

 able as was formerly supposed. 



Karsten (1906) confused this species with P. ele- 



gans Cleve, a species related to P. grande Kofoid. Kar- 

 sten's records of P. divergens var. elegans , as he states 

 later (1907), refer to P. oceanicum VanhOffen. Cleve 

 (1900b) also confused these two forms. 



Kofoid (1907a) described a form, P. murrayi , as 

 "resembling^, oceanicum " but with "lower epitheca 

 with more concave sides, longer horn, and longer and 

 more divergent antapical horns." A figure was given, 

 and the length given as 250 microns and the width 135 

 microns. This form seems to have a certain constancy 

 and is probably specifically distinct. It was figured lat- 

 er by Pavillard (1931) and Matzenauer (1933). It is, how- 

 ever, difficult to distinguish Pavillard's P. murrayi var. 

 occidentalis and Matzenauer's P. murrayi var. orien - 

 talis from P. oceanicum Vanhoffen. 



Distribution . This is a widely spread species. It 

 has been frequently reported from various parts of the 

 Atlantic (see Paulsen, 1908; Lebour, 1925; and Pavil- 

 lard, 1931) and from the Mediterranean (Forti, 1922). 

 Karsten (1905) reported it from the Antarctic. Karsten 

 (1907) and Matzenauer (1933) recorded it in the Indian 

 Ocean. The only records for the Pacific are those of 

 BOhm (1936) between Hong Kong and Shanghai. 



In the Carnegie collection this species was found at 

 9 stations, all in the Atlantic. There are 17 records of 

 occurrence: 6 rare, 5 occasional, 5 common, and 1 a- 

 bundant. It was found about equally at the three levels, 

 with 6 records for the surface, 5 for 50 meters, and 6 

 for 100 meters. There are 14 net records and 3 pump 

 records. All the records are from July and August 1928. 



The species was found at all stations between Ice- 

 land and Labrador, at two stations in the North Atlantic 

 Drift (stations 15 and 16), and at one station in the North 

 Equatorial Current, station 21 (fig. 20). It was most a- 

 bundant in the area between Iceland and Labrador. The 

 fact that it was not found on the run from Washington to 

 Plymouth during May might indicate that this particular 

 occurrence was seasonal. The complete absence of the 

 main species from the Pacific is very remarkable. All 

 the Pacific material could definitely be placed in var. 

 tenellum because of the shorter length. 



The surface temperatures at tht stations where P. 

 oceanicum occurred at any depth varied from 8°.4 to 

 26°.0 C. The ranges of hydrographic conditions in situ 

 were as follows: temperature, 4'iO to 26°6; salinity, 

 34.7 to 36.8 o/oo; pH 7.90 to 8.32; phosphate, 4 to 78 mg 

 P04/m3. 



It is obvious from these observations that this spe- 

 cies is not restricted to water masses of particular 

 characteristics even though its center of abundance is 

 definitely in the colder regions. 



Peridinium oceanicum var. tenellum n.var. 

 (Figures 32, 33) 



Dimensions . Length of body (\) 179 (150-205) mi- 

 crons. Diameter (d) 91 (70-106) microns. Width of gir- 

 dle about 7 microns. Twenty-seven specimens meas- 

 ured. 



Shape . Similar to main species but less slender. 

 The h/d ratio is 1.40 (1.25-1.76). The angle a is 126° 

 (110°-142°). The a/d ratio is 0.58 (0.42-0.67). Theb/a 

 ratio is 0.12 (0.08-0.16). 



Plate pattern . Eplthecal tabulation is of the first 

 symmetrical pattern. Other areas of theca also as in 

 main species. 



