1918] Barrows: Skeletal Variations in the Genus Peridinium 403 



spines, or by a ridged reticulation with spines at the intersections of 

 the ridges or even with papillae so large as to give a corrugated ap- 

 pearance to the shell {e.g., Peridinium thoriamim). In addition to 

 external markings, there is much evidence of the frequent if not uni- 

 form occurrence of perforations in the plates and of the existence of 

 a thin membrane of protoplasm outside of the plates. 



The protoplasm of many species carries chromatophores of yel- 

 low, brown, or green, and oil globules which may be associated with 

 the phosphorescence of many species under certain conditions. 



Nearly all members of the family, Peridinidae, are markedly 

 asymmetrical, both laterally and dorso-ventrally. In Triposolenia 

 Kofoid (1910) has shown that this asymmetry is of undoubted benefit 

 in so orienting the organism as it falls through the water at periods 

 of quiescence as to cause it to present as great a surface as possible 

 in the direction of the fall, thus retarding its rate of falling. The 

 asymmetry of man}' species of Peridinium and of other genera may be 

 explainable upon a similar basis. 



The anterior or posterior end of a dinoflagellate may be produced 

 into hollow horns or may carry spines apparently of solid material 

 and supported hy lists in the manner of buttresses. In forms with 

 long, hollow appendages or horns, notably in species of Ceratium, 

 autotomy of these horns has been observed (Kofoid, 1908) and has 

 been explained as an adaptation to flotation upon sudden changes in 

 the density of the surrounding sea water. 



Among the dinoflagellates a number of features fortunateh^ com- 

 bine to make the genus, Peridinium, a favorable subject for such an 

 inquiry as this : 



(1) The great abundance of certain species of the genus, and their 

 wide distribution, affording a possibility for comparing representa- 

 tives of the same species from regions presenting a great variety of 

 conditions ; 



(2) The morphologic features of these organisms which lend them- 

 selves readily to a systematic comparison; 



(3) The nature of the environment itself off'ering a new point of 

 view on account of its differences from the land environment under 

 which these problems have usually been studied ; 



(4) The existence in an environment which is unusually stable 

 and free from rapid or extreme variations, and which from many 

 aspects seems to be as simple an environment as can be found in any 

 considerable area of the earth, and an environment which may be 



