1&I 
Plagellata and Primitive Alga. 
Peridiniales); and Ceratieae (Ceratium, Peridinium, etc.—this is 
the largest division of the group). In the Dinophyseae the shell is 
divided by a longitudinal suture into two subequal lateral portions 
the epitheca is much smaller than the hypotheca, the borders of the» 
annulus are funnel-like and the left-hand border of the sulcus is 
often developed into wings and spines. 
According to Schiitt, the Peridiniaceae are connected with the 
Gymnodiniaceae by the genus Glenodiniutn, and with the Proro- 
centraceae by the genus Ptychodiscus. In Glenodiniutn (Fig. 9, 9 to 
13) the shell is thin and structureless (not sculptured or perforated), 
and its differentiation into two valves and a girdle is only apparent 
when rupture occurs at liberation of the encysted contents. In 
Ptychodiscus the two valves have the same structure as in the 
Prorocentraceae, but the girdle is represented by a thin soft 
membranous ring-like band, while the sulcus is indicated by a 
depression on one valve and a narrow plate on the other. 
The results of recent work suggest considerable modifications 
of Schutt’s classification of the Peridiniales, and appear to afford a 
basis for phylogenetic interpretations very different from those put 
forward by that author in 1896. Our knowledge of the Peridiniales 
and allied groups has been greatly extended in recent years by the 
work of Apstein (1, 2), Borgert (13), Chatton (21, 22), Dogiel (41), 
Jollos (62), Klebs (68), Kofoid (71, 72), Lemmermann (75-85), 
Lohmann (86-88), Schilling (127, 128), and others; the literature 
is cited by Pavillard (115) and in various other general works. 
The view that the Peridiniales are related to the Flagellata 
appears to have been first put forward by Bergh (4), who pointed 
out the striking resemblances between Prorocentrnm and the Crypto¬ 
monads. Bergh also suggested that a form like Prorocentrum 
might have given rise to the Dinophyseae, in which the transverse 
groove is near the anterior end of the cell, and that the Ceratieae 
are derived from the Dinophyseae by progressive shifting backwards 
of this groove to an approximately median position. Biitschli (18), 
on the other hand, considered that in the evolution of the Peri¬ 
diniales shifting of the annulus had taken place from behind 
forwards; according to his interpretation of the structure of Proro¬ 
centrum, which is followed by Schiitt, the suture between the two 
valves is horizontal, and the insertion of the flagella lateral. Bergh 
and Biitschli agreed in regarding the simple shell-less Gymno- 
diniacese as derived by reduction from the typical shell-clad 
Peridiniales, and various other writers have adopted this view, as 
being a necessary consequence of the principle that the Peridiniales 
are of monophyletic origin. 
A much simpler interpretation is obtained if we regard the 
suture in the Prorocentraceae as being longitudinal and as 
corresponding with the longitudinal suture in the Dinophyseae, 
which ought perhaps to be separated as a distinct family—the 
higher Peridiniales (Schutt’s Peridiniaceae) would then fall into two 
families, Ceratiaceae and Dinophysidaceae. Including the two 
families recently founded by Klebs and by Chatton, the Peridiniales 
as a whole may be regarded as forming two distinct series, which 
it is here suggested are of independent origin from the Cryptomonads. 
