256 University of California Puhlicatio)is in Zoology. [Vol. S 



being about five microns, possibly in correlation with a constancy 

 in the size of the flagellum or range of its field of action. 



The manner in which the furrow is formed varies in different 

 species. In one, G. sphaeroidea (pi. 16; fig. 41), it is not at all 

 impressed into the body wall, in several {G. ceratocoroides, G. 

 milneri) the surface is merely flattened, in others {G. palustris, 

 G. scrippsae, pi. 16, fig. 38, and in the subgenus Fusigonyaulax) 

 it is slightly impressed, while in the great majority of species, as 

 in G. spinifera (pi. 16, fig. 39), G. polygramma (pi. 17, fig. 47), 

 it is quite deeply set into the body wall. 



The sides of the furrow are variously constructed. In non- 

 impressed and shallow types the sides are formed by high mem- 

 branous lists either without {G. sphaeroidea) or with (G. milneri, 

 G. ceratocoroides, G. hirostris, G. glyptorhynchus) ribs. In 

 nearly all of the deeply impressed types there are no membranous 

 lists, the edges of the furrow being formed by thick scarcely 

 salient shelves of thecal wall, as in G. polygramma, G. triacantha, 

 and G. diegensis. In a few cases, as in G. spinifera (all?) these 

 edges may bear low fins with short ribs. 



The form of the longitudinal furrow or ventral area also 

 differs in different species, largely in correlation with the form 

 of the girdle. The overhang and displacement give it a marked 

 sigmoid curvature in G. spinifera (pi. 16, fig. 39), G. digitale 

 (pi. 9, fig. 3), and G. scrippsae (pi. 16, fig. 38). In species with 

 displaced girdle, such as G. diegensis (pi. 13, fig. 24), a narrow 

 constricted region extends between the two ends of the girdle. 

 In polyhedral forms, G. polyedra (pi. 17, fig. 43), G. milneri, and 

 G. ceratocoroides, and especially in G. catenata, this area is a 

 widely expanded, vertical, straight tract within which the minor 

 skeletal elements, six in number, which compose it, can be 

 readily determined. In all except the polyedra and sphaeroidea 

 groups the ventral area is widened posteriorly, enormously so in 

 G. triacantha (pi. 11, fig. 13) and G. catenata, and considerably 

 in the polygramma series (pi. 17, fig. 47). In the subgenus 

 Steiniella (except in G. mitra) the distal enlargement is con- 

 siderable and rather abrupt (pi. 17, fig. 45) and in the G. spini- 

 fera series it is often spoon-shaped (pi. 16, fig. 39). The degree 



