1918] Barroivs: Skeletal Variations in the Genus Peridinhim 405 



Representative genera of the Gymnodinidae as at present considered 

 are Pyrocystis Murray, Gymnodimum Stein, Blast odinium Chatton 

 (parasitic), Oxyrrhis Dujardin, and Pouchetia Schiitt. 



The family, Peridinidae, contains the typical members of the 

 group and is characterized by the full development of the general 

 characters of the group, including especially a "well developed cuirass 

 made up of definite plates" (Minehin, 1912, p. 278) and two flagella, 

 one lying in a transverse groove or girdle, and the other trailing pos- 

 teriorly from a ventral longitudinal groove. It is with this division 

 of the dinoflagellates that we shall be principally concerned. The 

 representative genera of this division will be named later. 



The third family, Dinophysidae, containing a number of bizarre 

 forms, differs from the Adinida in possessing a well-defined girdle 

 often with very large girdle lists which may extend along the margins 

 of the longitudinal groove as well, and differs from both the Gym- 

 nodinidae and Peridinidae in having the shell divided into two lateral 

 valves by a sagittal suture. Each valve is in turn divided into one 

 large and one small plate by a portion of the girdle which usually lies 

 well toward the anterior end of the organism. Representative genera 

 are Dinophysis Ehrbg., Phalacroma Stein, and Ornithocercus Stein. 

 From the fundamental skeletal character of a sagittal suture, it is 

 possible that the Dinophysidae may be more closely related to the 

 order, Adinida, which contains evidently the most primitive and gen- 

 eralized forms of the group, than to either the Gymnodinidae or the 

 Peridinidae, and that the bizarre development of the lists, etc., is of 

 secondary morphologic importance and perhaps acquired rather late 

 in the phylogenetic history of this portion of the group. 



Nomenclature op the Plates 



In this paper the sj^stem of plate nomenclature devised by Kofoid 

 (1909, p. 40) has been adopted as being at once the simplest, most 

 logical, and most adaptable by extension to related genera, of several 

 which have been proposed and wliich have been summarized by Kofoid 

 (1909, p. 44). This sj^stem recognizes the girdle as a basis of refer- 

 ence rather than a hypothetical equatorial plane, with which the 

 girdle rarely exactly corresponds on account of its more or less spiral 

 course. The girdle is, moreover, the most prominent single feature of 

 the organism and it lies in such a position relative to the plates in 

 Peridinium and in related genera that it is especially convenient to use 



