16 



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



P. claudicans Paulsen (1907) 



P. polymorphum Lindemann (1924) 



P. oceanicum VanhOffen (1897a, 1897b) 



P. oceanicum f. arupinense Broch (1910b) 



P. oceanicum var. parvulum Mangin (1913) 



P. oblongum Aurivillius (1893) 



P. oblongum var. symmetricum Dangeard (1927a) 



P. oblongum var. inaequale Dangeard (1927a) 



P. oblonecum var. iatidorsale Dangeard (1927a) 



P. obliquum Dangeard (1927a) 



P. murrayi Kofoid (1907) 



P. murrayi var. occidentals Pavillard (1931) 



P. murrayi var. orientale Matzenauer (1933) 



In the present report only those forms occurring in 

 the Carnegie collection are treated. They are: 



P. depressum Bailey 



P. depressum var. parallelum Broch 



P. depressum var. rectius n.var. 



P. depressum var. convexius n.var. 



P. depressum f. bisintercalares n.f. 



P. depressum f. multitabulatum n.f. 



P. claudicanoides n^sp. 



P. oceanicum Vanhoffen 



P. oceanicum var. tenellum n.var. 



P. oceanicum f. spiniferum n.f. 



P. oceanicum f. bisintercalares n.f. 



P. oceanicum f. tricornutum n.f. 



Methods of stud y. The first well-considered clas- 

 sification of the Peridlnium depressum group was based 

 on the principles outlined by Jorgensen (1913) for the ge- 

 nus as a whole. This classification presupposes a sta- 

 bility In the number and arrangement of the major plates 

 of the theca. The inadequacy of the last assumption 

 when applied to the Peridinium depressum group will be 

 clear from the following historical review and consider- 

 ation. 



Jorgensen (1913) described the genus Peridinium as 

 having the following pattern: 4ap, 3a, 7pr, 5po, 2ant. 

 Lebour (1925) and Paulsen (1931) broadened the concept 

 of the genus. They included JOrgensen's Archaeperidi - 

 nium, which has only two intercalary plates, and divided 

 the genus into two subgenera: Peridinium with three, 

 Archaeperidinium with two intercalary plates. In the 

 present investigations, forms in the section Oceanica of 

 the subgenus Peridinium were found with only two inter- 

 calary plates (pp. 21, 24); and Dangeard (1927b) report- 

 ed a variety with only one intercalary (see next para- 

 graph). Thus, this subgeneric character has been shown 

 invalid. 



From the Peridinium depressum group the follow- 

 ing examples of plate variation may be given. Barrows 

 (1918) described a specimen of P. oceanicum . which is 

 closely related to P. depressum . with the ventral epithe- 

 cal plates of the Paraperidinium type. Dangeard (1927b) 

 described what he called a variety of P. depressum with 

 the ventral epithecal plates of Paraperidinium and the 

 dorsal plates of the section Pyriformia . Considering 

 the body form and the Inclined girdle of this specimen, 

 along with its plate pattern, it is either a distinct vari- 

 ety or a species, unquestionably belonging to the group 

 related to P. depressum . Dangeard (1927b) also de- 

 scribed some aberrent plate patterns in P. oblongum . 

 These variants he designated as varieties, as follows: 

 var. symmetricum . with the usual dorsal epithecal pat- 

 tern of the section Oceanica ; var. inaequale, with the 

 pattern of the section Tabulata ; and var. Iatidorsale . 

 with only one intercalary plate (subgeneric character). 

 This single intercalary plate borders plates 2pr, 3pr, 



4pr, and 5pr according to his figure. In his table (p. 11) 

 he states that it also borders plate 6pr. 



Barrows (1918) was of the opinion that there are 

 only four unstable areas in the theca of Peridinium . 

 Variation, he maintained, occurs only in the areas at the 

 two sides of the first apical, and at the sides of the sec- 

 ond intercalary along with the adjacent plates involved 

 in any pattern. Other variable areas, however, were 

 found later. Peters (1928) described two patterns in P. 

 depressum in regard to the epithecal plates. 



The variation on the lateral sides of the epitheca in- 

 volves plates 2ap, 2pr, la, and 3pr on the left side and 

 4ap, 6pr, 3a, and 5pr on the right side (fig. 9). The var- 

 ious patterns will be referred to in the pages to follow 

 as the first and second symmetricalj and the first and 

 second asymmetrical patterns. In the first symmetri - 

 cal pattern . 2ap touches 3pr, and 4ap touches 5pr (fig. 

 9A). In the second symmetrical p attern . 2pr touches la, 

 and 6pr touches 3a (fig. 9B). In the first asymmetrical 

 pattern . 2pr touches la but 4ap touches 5pr (fig. 9C). In 

 the second asymmetrical pattern . 2ap touches 3pr but 

 6pr touches 3a ^fig. 9D). 



Peters (1928) reports all four of these patterns in 

 P. depressum . Lindemann's (1925, figs. 10, 11) figures 

 of P. marinum (=P. depressum ) show the first symmet- 

 rical and the first asymmetrical patterns. In the Car - 

 negie material they were not all found in the same spe- 

 cies. In P. depressum the first symmetrical and first 

 and second asymmetrical were found; in P. oceanicum 

 the first symmetrical and first asymmetrical; in P. 

 claudicanoides the second symmetrical and first asym- 

 metrical. 



These patterns are not stable features of the theca. 

 The length of the critical sutures between the four plates 

 in question on each side of the shell varies. In species 

 where more than one pattern occur, there is an inter- 

 gradation of the different patterns. Specimens occur in 

 which the edges of the four plates meet at a common 

 point, representing an intergrade between two patterns. 

 Lebour (1925, pi. 23d) shows this condition on both sides 

 of the theca, but this may be an error in drawing. Pe- 

 ters (1928) found the common intersection on each side 

 of the theca in different specimens. In the Carnegie ma- 

 terial this condition was found only in P. depressum and 

 only on the left side (fig. 9E). These observations upset 

 Barrow's contention that the edges of four plates never 

 meet in a common point and that an intergradation never 

 occurs between two alternate plate patterns (1918, pp. 

 453, 455). 



In order to study this intergradation and present it 

 graphically, measurements were made of the critical 

 sutures in species in which the first symmetrical and 

 first asymmetrical patterns occur. The following com- 

 putation was made: The length of the critical suture on 

 the left side was divided by the length of the critical su- 

 ture on the right side and the quotient called the x- ratio . 

 In the symmetrical pattern the ratio was considered a 

 positive quantity and in the asymmetrical a negative 

 quantity. Obviously, in the case of the intergrade pat- 

 tern (fig. 9E) the ratio was zero. 



The x-ratio was computed for 144 specimens of P. 

 depressum. P. oceanicum , and varieties selected at ran- 

 dom. The ratios were grouped into 0.10 unit classes and 

 the frequency of the classes plotted (fig. 10). It is evident 

 that in the Carnegie material the symmetrical was the com- 

 moner pattern, occurring in about 87 per cent of the 

 specimens. The commonest x-ratios were from +0.5 to 



