96 University of California Publications. [Zoology 



tion in the axis of locomotion is a common phenomenon among 

 organisms. It may be that the dorsal deflection of the head bal- 

 ances the deficiency in the length of the dorsal antapical. 



The balanced relationship of the two antapicals to each other 

 and to the midbody is as striking as the similar relationship of 

 the anteriorly deflected horns of the long-horned forms of Cera- 

 timn such as C. vultur Cleve, and C. patentissimum Ost. and 

 Schm. This balance is shown in their approximately equal de- 

 velopment and similar spread from the axis and in the corre- 

 spondence in the reversed curvatures of the two horns. This 

 balance is only approximate, however, appearing at its highest 

 development in T. tritncata and T. bicornis and progressively 

 encroached upon in T. fatuJa and T. ambulatrix, where the dorsal 

 horn is deflected dorsally without corresponding ventral deflec- 

 tion of the ventral horn. It is noteworthy that in both of these 

 species the dorsal deflection of the anterior process is decreased 

 as if to counterbalance the increased dorsal deflection of the 

 dorsal antapical. 



This coordination in the matter of the proportionate develop- 

 ment of the three processes from the midbody and the balanced 

 relationship of the antapicals throughout the species of the genus 

 suggests an important function in orientation for flotation and 

 locomotion. 



The body as in other Dinophysidae is composed of two valves, 

 a right and left, which separate along a sagittal suture line. The 

 suture in Triposolenia is not structurally marked except by a 

 faint line, or a few scattered pores along the ventral side of the 

 neck or in the tubercles on the major flexures of the antapicals. 

 The epitheca as in Amphisolenia is reduced to a small dome above 

 the girdle, while the hypotheca constitutes the greater part of the 

 organism. Neither girdle plates nor suture are apparent. 



The midbody (mb. PI. 15, fig. 1) is a centrally located enlarge- 

 ment, strongly compressed laterally, and having a triangular, 

 elliptical, oblong, or ovoidal outline in lateral view. Its altitude 

 is about 0.25, its transdiameter 0.1-0.04, and its dorso-ventral 

 -diameter 0.35-0.12 of the total length. It is broadly rounded on 

 the margins, which are three in number, an antero-dorsal, an 

 antero-veutral, and a post-margin, the three being subequal in all 



