414 University of California Publications in Zoology [Vol. 18 



Other Skeletal Characters. — This present trend to regard the 

 skeleton of dinoflagellates as of greater and greater value seems to 

 reflect the more and more nearly complete recognition of the morpho- 

 logical importance of this part of the structure of these organisms. 

 Nor is it unreasonable to look among the dinoflagellates, as we do 

 among almost all other groups of animals which possess skeletal struc- 

 tures of any sort, both internal and external, to the hard parts of the 

 organism for the reflection of the deep-seated processes of change. 

 Hard parts of organisms seem in all groups to be modified but slowly 

 either by hereditary influence from generation to generation or by a 

 possible impress of the environment. The fundamental connections 

 of related animals seem to be impressed upon the more permanent of 

 the structures. Particularly^ is this the case when the hard parts are 

 not composed of excreted extraneous material, but are retained in a 

 close relation to the living protoplasm of the organism. 



Of the characters of the skeleton of Peridinidae there are several 

 which may contribute more or less to an understanding of the rela- 

 tionships of the members of the family. The nature of the autapical 

 horns has already- been discussed and the inadequacy of this character 

 to serve even in first instance as a common divisor for the genus, 

 Peridinium. 



Size is of course an important character, but is so dependent upon 

 the extremely variable metabolic processes involved in the whole 

 course of food a.ssimilation and of excretion as to be hardly trust- 

 worthy to reveal far-reaching generic relationships. 



Shape is an important factor, but in view of the well-known 

 capacity for autotomy among the dinoflagellates and for individual 

 responses to protoplasmic pressure, the shape of an organism seems 

 to be too intimately under the control of the environment and is not 

 to be trusted to display an all-pervading set of relationships. 



Characters of the surface of the skeleton are also of a certain 

 value, and with the shape should be taken into consideration in the 

 confirmation of relationships proposed on any other basis, but this 

 character again is under direct influence not only of the environment 

 but also of age because the degree of the development of surface 

 markings is known to vary widely not only between species but also 

 between individuals of the same species presumably of different ages. 



Conclusions upon the Importance of Plate Relationships. — The 

 number of parts of which the skeleton is composed seems, however, 

 to be a much more fundamental thing than any of the characters just 



