356 University of California Publications. [| ZOOLOGY 
Dimensions :—length, 116-120; transdiameter, 100u; dorso- 
ventral, 754; furrow, 8 in width. 
Taken in vertical hauls from 40—95 fathoms to surface off San 
Diego in May and June. 
This species is not closely related to any described species. Its 
asymmetry is noticeable but no other adaptations to flotation in 
the theca were found in the individuals examined. 
Heterodinium rigdenae sp. nov. 
Pl. 18, figs. 6-8. 
A small pentagonal Peridinium-like species with a coarse 
polygonal reticulum and slight obliquity of the girdle, resem- 
bling P. acutangulum Lemm. 
Body pentagonal in face view with straight or nearly straight 
subequal sides, broadly bifurcated posteriorly with short stout 
conical antapicals. Length 1.3 and dorso-ventral diameter 0.6 
times the transdiameter. Epitheca without apical horn, the 
sides sloping in a straight line from the apical pore to the girdle, 
compressed dorso-ventrally and somewhat excavated in the mid- 
ventral region, lateral sutures prominent and doubled. Hy- 
potheea also compressed, the right margin concave, the left with 
projecting angle at the junction of postmedian and antapical 
plates. Ventral face excavated. The antapical horns are short, 
0.2 transdiameter in length, which is nearly equal to the slightly 
eurved margin which separates their bases. Their ends blunt 
with short terminal spinule. The girdle is inclined about 15° 
postero-ventrally from the equatorial plane. The transverse fur- 
row is deeply indented, more so towards its anterior than its 
posterior margin. It forms a descending right spiral with dis- 
placement slightly exceeding its width. Its distal end is feebly 
developed, the posterior ridge vanishing on the ventral face. 
The anterior ridge is a heavy projection of the body wall, nearly 
twice the height of the posterior. The longitudinal furrow is 
narrow and shallow, dilated posteriorly and extends from the 
