1918] Barrows: Skeletal Variations in the Genus Peridinium 399 



less rapidly and through a narrower range of fluctuation than in land 

 environments. The physical elements into which the marine environ- 

 ment may be analyzed — temperature, salinity, density, viscositj^ light 

 relations, gas content of the water, ocean currents, etc. — are compara- 

 tively^ few, and variations in each of these may be accurately measured. 



While the physical conditions of a marine environment are thus 

 comparatively simple, the absence of concrete barriers in the sea in- 

 troduces a complexity into the study of the conditions of life for 

 marine organisms not encountered among land organisms. The con 

 tinual mingling of waters of all oceans, due to surface currents and 

 the mixing of surface waters with bottom waters as well, introduce 

 factors of unprecedented complexity in a study of the biology of 

 marine organisms. Hence, we have to deal with conditions which 

 introduce at once the possibility of structural as well as functional 

 responses to the impact of the environment. 



The system of variations suggested in the dinoflagellates, especially 

 tangible in the Peridinidae, thus offers peculiar attractions for stud3^ 

 It is the purpose of this paper to describe briefly the systematic posi- 

 tion of the genus, Peridinium, in the group, Dinflagellata, and among 

 the Protozoa; to discuss the morphology of the theca of the genus, 

 Peridinimn, and the relations on this basis of Peridinium to other 

 genera of the group, Dinoflagellata, and from this discussion to bring 

 out the peculiar relations of the patterns of the plates of the theca ; to 

 suggest the significance of variations in the relative sizes of these 

 plates and of their arrangement into patterns which may possibly 

 give a clue to the sequence of species in this group ; and to suggest 

 the possible existence of unit characters in the sizes of certain plates. 

 and the coupling of these characters according to the permutations 

 permitted b}^ the geometrical relations of the plates themselves in the 

 formation of species. 



It will be suggested that the skeletal characters in this group 

 apparently may vary independently of one another, or under other 

 circumstances that they may be linked in groups ; that certain char- 

 acters apparently vary at random about a norm, though there seems 

 to be a progressive advance in a general direction of specialization; 

 that certain characters are evidently accompaniments of old age; and 

 that of fluctuating factors of the environment the temperature of the 

 water and its buoyancy may perhaps be the strongest stimuli for the 

 alterations of structure. 



