1918] Barrows: Skeletal Variations in the Genus Peridinium, 411 



character of the type of horn seems an insufficient basis, then, for the 

 differentiation of all the species of the genus. From its variable de- 

 velopment, moreover, throughout the genus the type of horn seems 

 not to represent a sufficiently profound condition in the organization 

 of the genus to merit recognition as a feature capable of dividing 

 the genus into two morphologically correlative portions. And it is 

 hardly to be supposed that in the presence of so many prominent 

 variable characters as those of which Peridinium is possessed a single 

 one might have been selected to direct the progress of evolution into 

 two paths. Though perhaps a practical convenience in taxonomy as 

 affording a basis for the ready identification of specimens, this char- 

 acter of the type of antapical horn can hardly be presumed to have 

 monopolized the early differentiation of the genus. 



Displacement of the Girdle. — Although the relations of the girdle 

 are undoubtedly correlated with the function of locomotion in a very 

 important way, it has not as yet been demonstrated that rotation of 

 these organisms is dextrad in certain species and sinistrad in others 

 according to the direction of deflection of the girdle, nor in which 

 direction this rotation takes place in those species in which the girdle 

 ends meet approximately opposite each other. Variations in the 

 amount of deflection also occur even in the same species. We are not 

 dealing here with the character of the presence or the position of the 

 girdle itself but merely with the relations of the ends of the girdle. 

 On the whole it does not seem probable that a character of such lim- 

 ited extent in the morphology of these forms could have assumed a 

 dominating influence in effectually dividing the genus into two cor- 

 relative parts. 



As a practical aid in gaining an acquaintance with the genus, 

 however, this character has been of considerable value. Early classi- 

 fiers, Schiitt, Bergh, and Gran, divided the genus abruptly into two 

 portions on the basis of the deflection of the distal end (right end 

 from viewpoint of the organism) of the girdle anteriorly or posteriorly 

 with respect to the proximal end through which passes the base of 

 the circumferential flagellum. Coupled with the character of an- 

 terior deflection in the division of the genus called Protoperidinium 

 was placed the character of the solid buttressed antapical horns, and 

 with the character of posterior deflection was coupled the character 

 of the hollow antapical horn to define the remaining portion of the 

 genus, Euperidinium. This pairing of characters, however, does not 

 alwavs hold throughout the genus. Moreover, in many species it is 



