94 University of California Publications. [Zoology 



an antapical prolongation. The distinctive characters of the 

 genus are (1) the form of the midbocly, which is strongly com- 

 pressed laterally, and (2) the presence of a pair of nearly equal 

 and usually nearly symmetrical antapical horns, both of which 

 arise from the midbody. 



The axial relations in Triposolenia differ from those of other 

 Dinophysidue owing to the obliquity of the neck and to the dorso- 

 ventral asymmetry of the antapicals. In Binopliysis and Phala- 

 croma the main (antero-posterior) axis passes through the center 

 of the epitheca, the center of the hypotheca and the antapex in 

 nearly a straight line. The bilateral asynnnetry in these genera, 

 if any, is slight. In Amphisolenia the main axis is straight, or 

 nearly so, except in some species in the distal part of the single 

 antapical horn. This is often bent to the left distally or curved 

 in the dorso-ventral plane in bifurcated or branched species such 

 as A. thrinax and A. bifurcata. The epitheca, neck, midbody, 

 and a large part of the antapical horn lie in the straight line of 

 the main axis. The species of the genus are thus predominantly 

 linear in type. 



In Triposolenia, however, the main axis is not a straight line, 

 for there is both bilateral and dorso-ventral asymmetry. Assum- 

 ing the axis of the midbody as a base of reference because of its 

 relation to the balanced antapicals and to the plane of the girdle, 

 we find that the anterior process is always displaced ventrally, in 

 T. raviiciformis and T. exilis to the very margin of the midbody, 

 in other species to a very slight extent. Its axis, moreover, is 

 always inclined 10°-20° ventrally from the main axis. The neck, 

 on the other hand, is displaced dorsally on the anterior process 

 and is inclined dorsally from 10°-35° from the axis of the mid- 

 body and is in addition not infrequently curved in the dorsal 

 direction. The plane of the girdle is nearly perpendicular to the 

 axis of the midbody, at least more nearly so than to the axis of 

 the neck. Presumably the axis of rotation in locomotion is ap- 

 proximately perpendicular to the plane of the girdle and is there- 

 fore coincident with or at least nearly parallel to the axis of the 

 midbody. 



The antapical extensions of the body among the Dinophysidae 

 are feebly developed except in Amplnsolenia and Triposolenia.. 



