FAMILY PERIDINIACEAE 



15 



Methods of Study 



Selection of material . Since it would have been an 

 insurmountable task to treat a really representative 

 number of species of this large genus, a compromise 

 plan of study had to be devised. For the sake of render- 

 ing an initial contribution to the exact knowledge of the 

 thecal morphology of Peridinium , representatives from 

 each of the major groups of the genus were selected for 

 detailed dissection, viz., P. depressum from Orthoperi- 

 dinium . P. crassipes and P. truncatum from Metaperi - 

 dinium, and P. pallidum from Paraperidinium . The re- 

 sults of these investigations give some information a- 

 bout both the generic unity and the interrelationships of 

 the lower than generic groups. 



In addition to these detailed dissection studies, a 

 statistical analysis was made of Peridinium depressum 

 and of its relatives comprising the Formenkreis Peri - 

 dinium depressum of Broch (1906). This work was un- 

 dertaken in order to establish a method of attack on a 

 very variable group. Thus, these studies have a three- 

 fold purpose: first, to acquire a knowledge of skeletal 

 mo-phology; second, to learn something of specific in- 

 terrelationships and generic unity; third, to inquire into 

 the nature of specific variability, a feature of exception- 

 al significance in this group. 



Measurements . In determining the size and in com- 

 puting ratios expressive of the shape of the Peridinium 

 body, certain measurements were used. Measurements 

 made with the specimen presenting the ventral face 

 (p. 1) are indicated schematically by figure 7. Desig- 

 nations of dimensions in that figure are as follows: a, 

 length of right antapical horn, considering the base of 

 the horn as a curved continuation of the lateral body 

 contour running to the base of the right side of the sul- 

 cus; b, thickness of the right antapical horn measured 

 midway between its base and apex; d, diameter of the 

 body at the girdle; h, distance between the end of the ap- 

 ical horn and the base of the right side of the sulcus; J, 

 total length, or distance between the ends of the apical 

 and right antapical horns; a, the angle representing the 

 degree of convexity of the lateral profile. The angle oc 

 is constructed with its vertex in the girdle at the right 

 edge of the body, and with its sides intersecting the body 

 walls at points at a distance from the vertex equal to 

 one-fourth of d. 



Measurements made with the specimen presenting 

 the apical view are indicated schematically by figure 8. 

 Designations of dimensions in that figure are as follows: 

 g, dorsoventral diameter measured through the mid- 

 body; r, in some species, e.g., P. truncatum . it is nec- 

 essary to measure the prolongation of the ventrolateral 

 limbs of the body in the equatorial region. The value r 

 is obtained as follows: Construct a line passing through 

 the apex (b) and the farthest point on the right limb (a). 

 On this line erect a perpendicular at point c. The point 

 c is determined by its distance from a which equals one- 

 third g. The thickness of the limb, r, is measured along 

 this perpendicular. 



Various features of the body shape can be expressed 

 by the dimensional proportions. The best reference for 

 most of these ratios is the diameter (d). It can be meas- 

 ured in either the ventral or the apical view and is in it- 

 self a good standard for the absolute size of the speci- 

 men. 



The general body form can be expressed by the 



angle a and the ratio of the length to the diameter of the 

 body. In computing the latter ratio, h, was frequently 

 used instead of, or in addition to, i because a slight 

 displacement of the specimen sometimes caused more 

 error in l_ than in h on account of the divergence of the 

 right antapical horn in some forms. The distance h 

 does not include the antapical horn, which has a tenden- 

 cy to vary more than the apical horn, so that the value 

 h/d gives a more accurate criterion of the length of the 

 body than does the ratio _l/d. 



In expressing the relative length of the antapical 

 horns, it is usually necessary to measure only one of 

 these structures, as the variation is usually proportion- 

 al. In this work the length of the right horn was meas- 

 ured, as it usually presents a truer value because of its 

 lesser degree of inclination in the ventral view. The 

 length relative to the diameter of the cell, a/d, was then 

 computed. For the relative thickness of the antapical 

 horns the ratio b/a, the thickness of the right antapical 

 horn in relation to its length, was computed. 



Where the shape of the body in apical view was im- 

 portant, the g/d ratio was computed. The thickness of 

 the lateral limbs in apical view was found to be best ex- 

 pressed by the ratio r/g. 



The width of the girdle was always measured and 

 many smaller measurements referred to this. 



Peridinium depressum Bailey and related forms 



Introductor y remarks . Broch (1906) designated as 

 the "Formenkreis Peridinium depressum Bailev" a 

 group of highly variable and intergrading forms closely 

 related to P. depressum Bailey. This species is an 

 Orthoperidinium with a four-sided second anterior in- 

 tercalary plate. The members of the group present one 

 of the most difficult taxonomic problems of the Peridi- 

 neae. They have been studied by several investigators, 

 whose results have been at variance and decidedly con- 

 fusing. Because of this fact, and since the Carnegie col- 

 lection was rich in these forms, this group was subject- 

 ed to an intensive analysis in order not only to solve the 

 problems of this particular group, but also to formulate 

 methods and concepts which might be applied to the ge- 

 nus as a whole or to other groups of the Peridineae 

 which present a series of variable and intergrading u- 

 nits. 



Diagnosis . The group as conceived in this report 

 includes all the species of Orthoperidinium in which the 

 plane of the girdle is inclined to the longitudinal axis. 

 All these species are confined to Jorgensen's section 

 Oceanica . They are, indeed, the only forms now known 

 in the entire genus with a strongly inclined girdle, ex- 

 cept P. amplum Matzenauer. In the latter species, how- 

 ever, the apical and antapical horns do not lie in the 

 same plane, a feature characteristic of the "Formen- 

 kreis." The species in this group are mostly large, u- 

 sually with prominent horns, and the girdle is left-hand- 

 ed. 



Extent of the group . The following forms have been 

 described in the literature: 



P. depressum Bailey (1855) 



P. depressum f. brevisulcatum Dangeard (1927a) 



P. parallelum Broch (1906) 



P. saltans Meunier (1910) 



P. antarcticum Schimper (Karsten, 1905) 



