362 University of California Publications in Zoology. [Vol. 4 



and appears occasionally in most of the species having elongated 

 slender horns. I have observed it, for example, in C. longipes, 

 C. intermedium, C. protuherans (fig. 20), C. macroceros, C. os- 

 tenfeldi, C. vultur, C. inflexum, C. carriense, C. tripos, C. arcua- 

 tum (figs. 11. 12, 26), C. schranki, C. azoricum and C. hetero- 



Figs. 16-19. — Ventral views of Ceratium trichoceros showing progressive 

 and proportionate reduction of the antapical horns in autotomy. X 220. 

 The form of the end of the horns after autotomy is shown under greater 

 magnification in supplementary sketches in connection with figure 17. 



camptum. It occurs also in the long-horned members of the sub- 

 genus Amphiceratium such as C. extensum and C. biceps (figs. 

 21-24). It is, however, relatively rare among the species of the 

 subgenus Biceratium such as C. furca, C. lineatum and C. eugram- 

 mum, where the antapical horns, and the apical also as a rule, 

 are relatively short and play a less important part in the economy 

 of flotation than they do in the long-horned species of the sub- 

 genera Euceratium and Amphiceratium. 



