FAMILY PERTDINIACEAE 



31 



its middle part it forms tlie rigiit margin of flagellar 

 pore. It is definitely raised to a position equivalent to 

 that of a hypothecal plate. In its anterior half it has a 

 dome- shaped prominence which rises over the flagellar 

 pore (fig. 41). At the middle of the left side of this 

 plate, at the posterior end of the flagellar pore, there is 

 a process which projects into the cell body (fig. 39F). 

 Between the posterior half of the left sulcal plate and 

 the posterior half of the right sulcal plate there lies a 

 narrow plate about 4 girdle widths long, the posterior 

 accessor y sulcal plate , the homologue of the very mi- 

 nute plate at the posterior end of the flagellar pore in P. 

 depressum and P. crassipes . Anteriorly it touches the 

 flagellar pore, and posteriorly, where it borders the 

 posterior sulcal plate, it is about 3 times wider than at 

 the anterior end. There is no detachable semicircular 

 plate such as occurs in P. depressum and P. crassipes . 



Body wall . Entire surface of the major body plates 

 and of all the sulcal plates, except the left and posterior 

 accessory sulcal plates, covered with a very prominent 

 coarse and regular reticulation. Plates lacking this are 

 entirely smooth. Reticulation so deep on apical and ant- 

 apical horns, especially in their distal parts, that it 

 gives the impression of "spinlness" along the edge of 

 the horns. Girdle plates with irregularly spaced trans- 

 verse ridges which may extend completely across the 

 girdle or only part way from the margin on either side. 

 The major body plates, the girdle plates, and the sulcal 

 plates are pierced by minute, irregularly scattered 

 pores. Arrangement of these pores bears no relation to 

 reticulations. Usually one or two pores per mesh; those 

 on the girdle plates not arranged in rows. 



Intercalary zones not found in our material, except 

 in two specimens. 



Lists . Girdle lists comparatively narrow, usually 

 less than 1 girdle width wide; strengthened by fairly 

 regularly spaced ribs, usually extending completely from 

 body to outer edge, but in some cases extending only 

 part way from body or from outer edge. 



The apical list system similar to that in P. depres - 

 sum . except for a lesser development generally and for 

 the absence of the lateral lists. The lists in this region 

 are only 0.25-0.50 girdle width wide. Apical list encir- 

 cles apex and is composed of three segments: the dor - 

 sal , right , and left. Dorsal segment attached to tip of 

 third apical and not joined by any lateral lists as in P. 

 depressum . P. crassipes . and P. pallidum . Right and 

 left segments are attached to apical and ventral edges of 

 second and fourth apical plates and continue down the 

 ventral side of the body only as far as the posterior end 

 of the ventral apical platelet (at the anterior end of the 

 first apical plate). Ventral apical platelet is set deeply 

 between these two lists (fig. 39C, E, H). 



Development of sulcal lists in this species is of par- 

 ticular interest. Left sulcal list is well developed in re- 

 spect to its width, which is from 1.0 to 1.5 girdle widths. 

 The list extends laterally over the left anterior part of 

 the sulcus, overlapping the anterior end of the right ac- 

 cessory list and flagellar pore. It is continuous with the 

 posterior girdle list and is attached to the first postcin- 

 gular plate. Therefore, it can extend only halfway down 

 the side of the ventral area. At this point it joins the 

 left accessor y sulcal list , which is just as wide as the 

 left list but projects ventrally from the body. It is at- 

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

 is continued around the posterior region of the sulcus as 

 the posterior accessory sulcal list . This list, which is 



attached to the posterior sulcal plate, is narrower than 

 the last mentioned list and joins the p osterior sulcal 

 list at the right posterior corner of the ventral area. 

 Posterior sulcal list is a poorly developed list along the 

 posterior margin of the ventral area. It is composed of 

 two segments; the left segment, which is attachedtothe 

 first antapical plate, and the right segment, which is at- 

 tached to the second antapical plate. Right sulcal list is 

 absent. In other species it runs along the right edge of 

 the right sulcal plate (attached, however, to body plates) 

 and defines the right margin of the sulcus proper. In 

 this species its absence coincides with the raising of the 

 right sulcal plate to the position of a major body plate. 

 The functional list to the right of the sulcus proper in 

 this species is the right accessor y sulcal list. It is at- 

 tached to the left edge of the right sulcal plate. It ex- 

 tends laterally over the flagellar pore and the posterior 

 part of the sulcus. Posteriorly it connects with the pos- 

 terior accessory sulcal list but has a free lobe extend- 

 ing posteriorly (fig. 39D, E). The posterior accessory 

 list extends along the anterior edge of the middle and 

 right parts of the posterior sulcal plate. 



Comparisons . This species is rather closely relat- 

 ed to P. elegans Cleve (a species not treated in this re- 

 port) but may be distinguished from the latter by its 

 greater expansion in the girdle region, much greater de- 

 velopment of the ventrolateral body limbs, trimcated 

 antapical horns, and shape of the second precingular 

 plate. The r/g ratio is 0.47, as compared with 0.84 for 

 P. elegans . The second precingular plate in P. trunca - 

 tum narrows toward the girdle, whereas in P. elegans it 

 widens. The girdle margin of this plate in P. truncatum 

 is less than one-fourth that of po6; in P. elegans that 

 margin is more than half that of po6. Furthermore, P. 

 truncatum is a larger species, with an h/d ratio on the 

 average greater than that of P. elegans . 



A comparison between the ventral areas of P. trun - 

 catum and P. depressum is of interest. Although the 

 areas are constructed on the same fundamental plan, 

 with all plates homologous, they show many marked dif- 

 ferences in pattern, contours, and lists. 



The sulcal plates in P. truncatum show a definite 

 tendency to become major body plates. Thus a smaller 

 number of plates form the sulcus proper in P. truncatum 

 than in P. depressum . In P. depressum the left limb of 

 the posterior sulcal plate is quite inconspicuous, where- 

 as in P. truncatum it is prominent and fits into the main 

 body complex posterior to pol. The right limb of the 

 posterior sulcal plate likewise is prominent in undis- 

 sected specimens of P. truncatum . whereas in P. de- 

 pressum it is quite obscure. The right sulcal plate in P. 

 truncatum is a prominent feature of the ventral aspect 

 of the hypotheca, whereas in P. depressum it is a com- 

 paratively obscure plate of the sulcus. In P. depressum 

 it has a trough running the length of it, whereas in P. 

 truncatum this plate is convex and actually humped in 

 the anterior half. In the sulcus of P. depressum there 

 is a right internal sulcal plate; in P. truncatum this is 

 absent. The posterior accessory sulcal plate is minute 

 in P. depressum . but in P. truncatum it is 4 girdle 

 widths long. 



The positions of the sulcal plates in relation to the 

 plates of the body differ considerably in the two species. 

 In P. truncatum the suture between po5 and ant2 reaches 

 the ventral area in the middle of the right sulcal plate, 

 whereas in P. depressum this suture runs much more 

 posteriorly and reaches the ventral area in the middle 



