180 



point, four apical plates (Fig. 4cf/> 1'— 4'). Plates 1' and 4' lie on the 

 vpiitral surface separated from each other by a midventral suture which 

 rises from near the apex of the ventral plate (Fig. 5). This suture is not 

 always easily seen, more readily, perhaps, in the G. fvms group (Fig. 4) 

 than in those of the C. furca (Fig. 1) group. It is also less readily se- 

 parable on treatment with reagents. This probable accounts for the 

 fact that in Stein's (1883) figures of C. furca this suture is omitted, 

 while it is given by him for C. fustis. The fact that these two plates 

 frequently adhere to each other after the other sutures of the apical 

 series separate has given rise to the earlier statements that Ceratium 

 has but three apical plates. These two plates are somewhat more slender 

 than the two on the dorsal side. The sutures which separate the ventral 

 and dorsal pairs lie on the ventro-lateral faces and are consequently 

 less easily followed in many species. These rest posteriorly upon the 

 precingular series of four plates, instead of three as usually reported 



Pig. 5. Ventral view of C. intermedium, region of the midbody only. X 350. 

 Fig. 6. Dorsal view of C. intermedium. X 3.50. 



(Fig. 4p-ec 1" — 4"). The line of fission, marked in all the figures by two 

 parallel lines divides these into a right and left pair on the middorsal 

 line. Precingular plates 3" and 4" of my nomenclatui-e seem to have 

 been regarded as a single plate by other investigators, except by Entz 

 (1905), by the omission of the suture separating them or failure to note 

 its significance when detected. 



The postcingular series (Fig. ^postc 1'"— 5'") is composed of 5 pla- 

 tes, of which two (1"' and 5'") are small and lie on the ventral face of 

 the hypotheca. Plate 1'" is always small and covers a relatively small 

 area posterior to and to the left of the flagellar pore, and generally has 

 a somewhat oblique posterior border. Plate 5'" forms the ventral fac^ 

 of the right antapical. Its presence is obscured by the fact that the 

 suture which separates it from plate 4'" lies in the frontal plane passing 

 from the girdle posteriorly along the lateral margin of the hypotheca to 



