RELATIONSHIPS AMONG THE GENERA 



plates in the hypotheca as in the epitheca ( Heterodlni - 

 um), some with more plates in the hypotheca ( Centro - 

 dinium), and the majority have more in the epitheca 

 than in the hypotheca. This is an expression of the well- 

 known principle of the independent evolution of particu- 

 lar organs or organ systems which is so amply illus- 

 trated throughout the Metazoa. 



In regard to the arrangement of plates it is even 

 more difficult to attempt a reconstruction of evolution- 

 ary trends than in the case of the number of plates. We 

 do not know of any forms which represent definite tran- 

 sitional stages between the unarmored Dinoflagellata 

 and the Peridiniales. Thus, we have no reasonably cer- 

 tain information as to which are the most primitive pat- 

 terns. As a consequence, we find ourselves forced to 

 undertake a search for features which are more or less 

 universal, and to start from the assumption that such 

 characters represent the primitive condition. 



The peridinian skeleton is divided naturally into 

 four parts and, for purposes of evolutionary discussions, 

 it is advisable to consider these separately. They are: 

 the epitheca, the hypotheca, the girdle, and the ventral 

 area. 



Because of the unreliability of many of the pub- 

 lished generic diagnoses, the present discussion of re- 

 lationships is based entirely on the forms treated in this 

 report. The skeletal features of these are known with 

 certainty. Since the ventral area has been the main tar- 

 get in the present work and, since the comparison of the 

 ventral areas has been the most fruitful in showing in- 

 tergeneric relationships, it will be taken up first. 



Ventral Area 



The ventral area was almost unknown in the Peri- 

 diniales when this investigation was started. Since then, 

 however, Abe (1936) has begun a reclassification of the 

 genus Peridinium on the basis of the structure of this 

 region, and Tai and Skogsberg (1934) have demonstrated 

 the importance of this feature in the Dinophysiales. 



A study of this area in various genera of the Peri- 

 diniales has shown that there is a rather small and fair- 

 ly constant number of plates, and that the shape and ar- 

 rangement of these plates differ fundamentally in the 

 different genera. 



General shape. There has been an extensive evolu- 

 tion of the general contours of the ventral area in the 

 Peridiniales. The area is seen in its simplest form in 

 Goniodoma . where it is comparatively flat and lacks in- 

 tricate structures. Furthermore, In this genus there is 

 no distinct delimitation of the sulcus proper. In Cerati- 

 um the ventral area is specialized, being broad and rel- 

 atively large, with extremely hyaline plates and with a 

 small sulcus on the left side. In the other genera inves- 

 tigated, there is a deep sulcal groove, in many cases so 

 deep and narrow that no indication of the tabulation can 

 be ascertained without dissection. In Spiraulax, Gon - 

 yaulax fusiformis , and Acanthogonyaulax the posterior 

 part of the ventral area flares out into more or less ex- 

 panded areas which cannot be considered part of the sul- 

 cus proper. In Peridinium this does not occur and the 

 sulcus proper is usually limited. In this case the sulcus 

 usually lies to the left side of the ventral area, although 

 there is considerable variation in this respect within the 

 genus. In P. truncatum , for example, the right sulcal 

 plate is on a level with the major body plates, as is also 

 the left limb of the posterior plate, and the left sulcal 



plate is turned in so that it forms the side wall of a 

 deeply embedded groove. 



In most of the forms of the Peridiniales represent- 

 ed in this report the two sides of the body are about 

 equal in size, and as a consequence, the ventral area 

 appears in or near the mid-ventral line when the organ- 

 ism presents the ventral view. In Gonyaulax pacifica, 

 on the other hand, a differential growth has taken place 

 by which the left side has grown decidedly larger than 

 the right. A consequence of this is that when the organ- 

 ism presents the ventral aspect, the ventral area lies 

 decidedly to the right and the axis of the sulcus has been 

 so rotated that only the plates of the right side are visi- 

 ble. 



Plate pattern. The ventral area in the forms investi- 

 gated is composed of from five to seven external plates. 

 (Fig. 1.) As regards homologues, however, there are 

 eight plates, as is shown by the fact that in Acanthogon - 

 yaulax , which has seven plates, there is a left acces- 

 sory but no posterior accessory; and in Gonyaulax pa- 

 cifica, which also has seven plates, there is a posterior 

 accessory but no left accessory. 



Thus, the generalized composite picture of the ven- 

 tral area has eight external plates. These are: an ante- 

 rior, a posterior, a left and a right, with accessories to 

 the left, right, and posterior, plus an intercalary in the 

 right posterior region. All border on the flagellar pore 

 except the posterior sulcal (Goniodoma excepted) and 

 the intercalary sulcal. In addition to these external 

 plates, in Peridinium there is an internal plate lying 

 along the right side of the pore. 



The anterior sulcal plate (as, fig. 1) forms the ante- 

 rior edge of the pore in all genera except in Acantho - 

 gonyaulax . where it has migrated anteriorly. The ante- 

 rior plate in all cases lies against the proximal end of 

 the girdle and in some genera it forms a distinct part of 

 the girdle, as, for instance, in Goniodoma . Frequently 

 this plate has a considerable extension into the epitheca, 

 viz., in Goniodoma , Gonyaulax . and Spiraulax . In Peri - 

 dinium the anterior plate bears part of the anterior in- 

 ternal process. It can be concluded that the anterior 

 sulcal plate is an important plate in the thecal complex 

 of the peridinian skeleton. It is a conservative plate, 

 occupying a key position at the head of the flagellar pore, 

 at the proximal end of the girdle, and in the epithecal 

 pattern. Only in Acanthogonyaulax is it removed from 

 its position at the head of the pore, but even in this ge- 

 nus it lies at the proximal end of the girdle. Its shape 

 varies considerably from genus to genus, but none of the 

 differences are radical. It may be squarish to linear 

 with various numbers of sides or irregularities. Fre- 

 quently the anterior cingular list is extended across it, 

 as in Goniodoma and Gonyaulax . 



The posterior sulcal p late (ps, fig. 1) is another im- 

 portant element. It is conservative in being always 

 present, but it has undergone a much greater diversifi- 

 cation than the anterior sulcal plate. It is always re- 

 moved from the flagellar pore except in Goniodoma . In 

 most genera it flares out into a relatively wide expanse 

 on the ventral aspect of the hypotheca. This expansion 

 may be elliptical ( Gonyaulax and Spiraulax ) or winged 

 (Acanthogonyaulax ) . In Ceratocorys it is rather obscure, 

 being rectangular and not wider than the anterior part of 

 the sulcus. It attains its greatest complexity in the ge- 

 nus Peridinium . where it is very intricate and lies at 

 the posterior end of a deep sulcal groove. In this genus 

 it Is more or less horseshoe-shaped with a narrow cen- 



