12 



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



only constancy in the entire pattern in Peridinium s. 1. 

 is the absence of posterior intercalaries and the pres- 

 ence of two antapicals. For abbreviations of the names 

 of the plates, see above, p. 3. 



In the forms treated in this report the pattern of the 

 major body plates is as follows: 



4ap (5 in P. depressum f. multitabulatum n.f.); 



3a (2 in P. depressum f. bisintercalares and P. oceani- 



cum f. bisintercalares n.f.); 

 7pr (8 in P. depressum f. multitabulatum n.f.); 

 4g; 5po; Op; 2ant. 



The four girdle plates are of unequal length. The 

 third is the longest, comprising all the dorsal and later- 

 al sides of the girdle and, in P. p allidum , most of the 

 ventral side also. The other three lie ventrally and are 

 comparatively small. The girdle plates may be very 

 significant in the classification of the genus, the number 

 being of generic value, and the pattern subgeneric. The 

 determination of the girdle plates has too often been 

 neglected in analysis of the thecal pattern, in spite of 

 the fact that their dissection is not difficult. 



The epitheca of Peridinium is in an unstable condi- 

 tion, as will be pointed out later. In all species dissect- 

 ed, however, there was found an apical ring platelet and 

 a ventral apical platelet. No variation was found in the 

 number of girdle plates nor in the hypotheca except in 

 P. oceanicum f. tricornutum n.f. 



Probably the most important region of the theca to 

 be investigated at the present time is the ventral area 

 or sulcal region. This is a very complex area made up 

 of several plates. The necessity of dissection in deter- 

 mining these structures has retarded our knowledge of 

 them. Kofoid (1909) reported on this region in P. steinii, 

 but it is not certain that complete dissections were made 

 and his analysis must be verified before being accepted 

 into the general concept of the ventral area of Peridini - 

 um . Faure-Fremiet and Puigeandeau (1922) indicated 

 plates in the ventral area of several species of Peridini- 

 um including P. oblongum and P. crassipes . They show 

 seven plates. These must have been drawn from the 

 authors' imagination, as they bear no relation whatever 

 to the actual tabulation of these species. The same may 

 be said for the sutures shown encircling the antapical 

 horns (fig. 9). The only detailed analysis of this area in 

 the genus so far published which is at all acceptable, is 

 that of Abe (1936) for Peridinium ovatum and related 

 forms. These analyses can be incorporated into the 

 general scheme developed from our own several analy- 

 ses in widely separate representatives of the genus. 



Certain features of the plate pattern of the ventral 

 area in the genus Peridinium are conservative; others 

 vary from species to species. The number of plates is 

 not constant in the genus, but varies from five to six; 

 different plates may be wanting. The conservative ele- 

 ments are represented by four plates: an anterior , a 

 posterior , a right, and a left sulcal plate. These plates 

 are conservative in that they are always present and 

 always have the same relation to each other and to the 

 flagellar pore. They do vary considerably in shape, 

 however, in the different species. The anterior sulcal 

 plate occupies the area between the ends of the girdle 

 and joins the posterior end of the first apical; it extends 

 to the anterior edge of the flagellar pore. The posterior 

 plate is a very complex structure, the most complex 

 plate so far found in the peridinian skeleton. Its shape 

 is roughly that of a U in which one limb has been rotat- 



ed so that its axis lies at right angles to that of the 

 other. The curve of the U is narrow, and the limbs are 

 expanded into wide areas. The details of the shape will 

 be described under each species. The right plate is the 

 largest of the ventral area and forms the right edge of 

 the flagellar pore. It extends from the anterior to the 

 posterior sulcal plate. It bears a prominent list on its 

 left margin, the right accessory sulcal list, which over- 

 lies the pore and may bear processes which project into 

 the protoplast. This plate is a prominent element of the 

 ventral area in undissected specimens. The left sulcal 

 plate is deeply set in the sulcus and is not clearly seen 

 without proper cleaning or dissecting of the specimen. 

 It is, however, fairly large and forms the left side of 

 the ventral area and the left edge of the flagellar pore. 

 It is one of the most constant elements of the area and 

 varies but little from species to species. In addition to 

 these four principal plates, the ventral area may have 

 three other plates, although all of these never occur 

 simultaneously. They are: the right accessor y sulcal 

 plate , the posterior accessor y sulcal plate , and the right 

 internal sulcal plate . The right accessory sulcal plate 

 lies against the distal end of the girdle. It was found 

 only in P. p allidum and it represents simply the anteri- 

 or end of the left sulcal plate in other forms. The pos- 

 terior accessor y sulcal plate lies between the right and 

 the left and the posterior sulcal plates, thus forming 

 part of the posterior margin of the pore. It occurs in 

 all the species, and is comparatively large in P. trunca - 

 tum (24 microns long) and extremely minute (about 2 

 microns long) in the other species investigated. The 

 right internal sulcal p late is a part of the complex inter- 

 nal skeleton on the inner side of the right sulcal plate. 

 It was found only in P. crassipes and P. depressum. 



The apex of the theca is quite complex, with the 

 same general structure throughout all the species inves- 

 tigated. Only three of the four apical plates actually ex- 

 tend to the apex; the first is subtended by a narrow 

 plate, the ventral apical platelet. The apical ends of this 

 and of the second to fourth apical plates are joined to- 

 gether around the apical pore platelet , which is a ring 

 embedded in the apex. 



Structure of thecal wall . Little is known of the de- 

 tailed structure of the thecal wall. It is quite thick in 

 all the species investigated. Its exact thickness is dif- 

 ficult to determine, but it is probably 1 to 2 microns in 

 the thinner parts and considerably more in the thicker 

 parts. The surface is variously marked with reticula- 

 tions or tubercles. All the species are porulate at least 

 in the larger plates; sometimes some of the sulcal 

 plates are entirely smooth. 



In the species investigated, all plates are fastened 

 together by rabbet joints, as has been shown for other 

 species by Peters (1928). This joint is constructed as 

 follows: Along each suture of the theca there is attached 

 to one of the two adjoining plates a strengthening mem- 

 brane which underlaps the adjacent plate, forming a 

 rabbet joint (fig. 3A). Thus there are an external and 

 and internal suture separated from each other by the 

 width of the membrane. These membranes are an inte- 

 gral part of the plate to which they are attached and can- 

 not be detached from it. They occur whether or not there 

 is an intercalary zone. When such a zone is present, 

 the external suture divides it approximately in the mid- 

 dle, so that when the two plates are separated, half of 

 the zone remains attached to the plate without the mem- 

 brane and half to the plate with the membrane. Since 



