166 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Prorocentrum curvatum, sp. nov. 



Plate 1, Figs. 1, 3. 



A small species with lanceolate curved body. 



Body elongated, its length (dorso-ventral axis) 3 times the transdiameter and 

 5.5 times the anteroposterior one. Ventral end widest, truncate, bearing a short 

 median flagellar collar which is anterior to the level of the suture. The body 

 has nearly straight lateral margins for 0.5 of the length, then tapers to a blunt 

 point. Seen from the side the body is curved posteriorly, the more distally, 

 till the dorsal apex is almost at right angles to the ventral axis. The posterior 

 valve is concave, and nearly flat, the anterior is convex both dorso-ventrally 

 and transversely. 



The thecal wall bears 6-7 longitudinal rows of close set pores on each valve. 



Chromatophores small, irregular, dark yellow. 



Length (dorso-ventral axis), 65 /x ; transdiameter, 22 jx. 



Station, 4720. 



Pyrocystis acuta, sp. nov. 



Plate 1, Fig. 4. 



A large species with slender, straight, or slightly concave, cylindrical body 

 swollen at the centre and abruptly pointed at the tips. The length is 13-21 

 times the diameter of the swollen midregion. The shaft beyond the midbody 

 is 0.35-0.6 of the greatest diameter. The taper to the acute point is confined 

 within one transdiameter of the end. The ends and the midbody differentiate 

 the species clearly from P. lanceolata. 



Length, 675-1400 /x ; transdiameter, 45-95 /x. 



Stations, 4728, 4732, 4740. 



Pyrocystis fusif ormis f. biconica, f. nov. 



Plate 1, Fig. 3. 



A small biconical form with broadly rounded apices and midregion. Appar- 

 ently intermediate in form between P. noctiluca and P. fusiformis but not 

 plainly in tergrading with either. The length is 1.4-2.75 times the diameter. 

 Differs from P. fusiformis in its relatively greater girth and in its straight rather 

 than convex sides. 



Length, 160-380 \i ; transdiameter, 60-215 /x. 



Stations, 4728, 4732, 4740. 



Pyrocystis semicircularis (Schroedee). 1 



Plate 1, Fig. 6. 



A medium-sized species with small ellipsoidal midbody and long slender 

 cylindrical tapering incurved horns. Often yoked in pairs, as in P. hamulus. 



1 Schroder, B. Beitrage zur Kenntnis des Phytoplanktons warmer Meere. Viert. 

 Nat. Ges. Zurich, 51, p. 319-377, 1906. Received while this paper was in press. 



