458 University of California Publications in Zoology [Vol. 18 



in which the pattern of Orthoperidinium characteristic of P. oceanicum 

 in its typical form has given way to the more primitive pattern of 

 Paraperidiniiitn. This striking reversion is accompanied also by an 

 abortion of the antapical horns. Other characters, and particularly 

 the relation of its position in the fauna of the collection in which it 

 occurred to the general population of the region, leave little doubt 

 that this particular specimen is a highly modified specimen of the 

 species P. oceanicum. In San Francisco Bay were taken the specimens 

 of P. claudicans which display the unusual right oblique dorsal pat- 

 tern. P. claudicans and P. oceanicum are both species of the open sea, 

 and it seems highly probable that the altered conditions of salinity, 

 density, and temperature encountered by those specimens swept into 

 bays by the tide may have stimulated these departures from the normal 

 structure of these species. 



C. CONCLUSIONS 



From this analysis of the morphology of the skeleton of Peridinium 

 the following conclusions are suggested: 



1. A distinct difference is to be pointed out not only in the organ- 

 ization of the shell in the two principal families of the Dinifera, the 

 Peridinidae and the Dinophysidae, but also in the manner of develop- 

 ment followed in each family. In the Dinophysidae the number of 

 plates remains constant. These four major plates seem to be able to 

 conform to a great variety of forms of the body without breaking up 

 into smaller plates. The skeletal variations in this family consist of 

 differences in the development of such skeletal appendages as the lists 

 of the girdle and of the longitudinal groove, and also in the degree of 

 porulation, distribution of pores, and association of pores with other 

 surface markings. 



In the Peridinidae, on the other hand, the skeleton behaves very 

 differently, and progressive development seems to proceed in an orderly 

 fashion in the direction of an increase in the number of plates of 

 wliich the skeleton consists, as well as in the development of various 

 features of minor importance, including porulation, surface markings, 

 and antapical horns. The plates become at once arranged in circum- 

 ferential rows parallel to the girdle, which is of dominant importance 

 in the activity of the organism. The family, Peridinidae, is to be 

 characterized especially by this capacity for a peculiar and orderly 

 fragmentation of the shell according to a progressive scheme. 



