402 University of California Puhlications in Zoology [Vol. 18 



defined depression, the longitudinal groove or sulcus, which is regarded 

 as ventral in position. In function the longitudinal flagellum imparts 

 the forward motion and the transverse flagellum imparts a rotary 

 motion. By means of this rotary motion spiral progression in a 

 fairly straight line is secured, which would otherwise be impossible 

 on account of the asymmetry of the organism. The margins of 

 these grooves, especially of the transverse groove, carry hyaline lists, 

 or wings, apparently supported by stiffened ridges or ribs. The pur- 

 pose of these grooves with their extended hyaline margins as recep- 

 tacles for the fla^ella is probably to serve as flanges to increase the grip 

 of the organism upon the water, and to make more effective the beat- 

 ing of the flagella, thus facilitating the rotation and locomotion. 



The transverse groove or girdle divides the shell into two distinct 

 portions, the anterior of which is known as the epitheca and the pos- 

 terior as the hypotheca. 



The shell of the dinoflagellates has been shown by analj^sis to be 

 composed of an ashless substance similar to cellulose. At certain 

 stages of the life of an individual this shell is homogeneous and un- 

 broken. Through a breaking down of certain parts of the shell along 

 pattern lines peculiar to each group and species, the shell is divided 

 into a number of plates. The cellulose substance seems to break down 

 at the suture lines of the plates and to be replaced by a cement which 

 yields to dissolving alkaline reagents which have little effect upon the 

 cellulose of the plates themselves. The joint at the sutures is beveled 

 and corrugated (Kofoid, 1909) and it seems probable that limited 

 growth with age is permitted by the widening of this beveled edge 

 through the addition of shell material on the margin of the plate. 

 The great differences in the appearance of the sutures varying in dif- 

 ferent individuals from narrow, highly refractive lines to wide, stri- 

 ated tracts are regarded as due to the widening of the sutures to 

 accommodate the internal pressure of growth with age. Significant in 

 this connection is a chain of Gonyaulax figured by Kofoid and Rigden 

 (1912), showing the increase in size with successive asexual divi- 

 sions from a single individual, as demonstrating the possibility of 

 increase in size in this method of reproduction, and hence the prob- 

 able potentiality of dinoflagellate protoplasm for growth without 

 actual division, provided means for alleviating the confinement of the 

 shell can be secured, such as growth of the plates on thin edges. 



The plates of the shell may be variously marked, though in many 

 species the shell may be nearly if not quite smooth, or covered with 



