191!^] Barrows: Skeletal Variations in the Genus Peridinium 401 



and by the possession of chromatophores in some surface forms, con- 

 stitute a group which is evidently bound together by intimate j)hylo- 

 genetic relationships. 



Among this vast assemblage of parasitic as well as free living 

 organisms included among the Mastigophora, the Dinoflagellata 

 (Biitschli) are the largest group which is found extensively in the sea. 

 The other free living Mastigophora are found largely in fresh water. 

 In the sea the dinoflagellates seem, however, to have multiplied in 

 great variety, doubtless originating there in an early geologic period. 

 Because of the purely organic nature of the shell of these organisms 

 dinoflagellates have been rarely found preserved as fossils, but a few 

 figures are given by Ehrenberg of specimens from horn stone (similar 

 to flint) from Delitzsch, Saxony (1854, pi. 37, fig. VII, 1, 3, 4) ; from 

 Pottschappel, Saxony (1854, pi. 37, fig. VII, 3, 4) ; and from coral- 

 crag formations of Krakow, Poland (1854, pi. 37, fig. VIII, 1), and 

 also from blackish-brown, leaf-coal measures of Westwalde, Gei*many 

 (1854, pi. 7, fig. II, 13-15). These, however, seem to have been so 

 poorly preserved in most cases as not to permit definite descriptions 

 or classification, though more or less of a plate pattern is discernible 

 from some of Ehrenberg 's drawings. 



Onl}^ a comparatively few forms of dinoflagellates have adapted 

 themselves to fresh water conditions, having been carried thither from 

 the sea or from brackish Avater areas, most probably by water birds 

 or in spray whipped into the air by the wind and deposited with 

 atmospheric dust (cf. Ehrenberg, 1854, pi. 7, fig. B, 3). These- fresh 

 w^ater forms suggest, from their close relations to the more highly 

 developed of the dinoflagellate types rather than to the more simply 

 organized and presumably more primitive types, a transition from the 

 sea at a comparatively recent geologic period. 



The dinoflagellates are, moreover, a fairly well known group. 

 Extensive investigations upon them date from the time of Ehrenberg 's 

 first descriptions in 1830. Because of their abundance and impor- 

 tance, they have received much attention and the literature dealing 

 with them has become very large. Hundreds of descriptions are 

 recorded among which there must be over six hundred valid species. 



The flagella in dinoflagellates commonly originate in the region of 

 a pore, located on what is termed the ventral side. One flagellum 

 circumscribes the body of the organism in a groove, the girdle, located 

 according to the genus near the anterior end or about the midregion. 

 The other flagellum trailing posteriori}^ lies in a more or less well 



