412 University of California Puhlications in Zoology [Vol. 18 



impossible to determine any deflection of the girdle. In practical 

 application Paulsen (1908) continued this basis of classification, sub- 

 ordinating to it the characters of presence or absence of antapical 

 spines, the type of spine, shape of the body, etc. 



Faure-Fremiet (1908) divided this genus into four groups upon 

 the basis of the total of resemblances including form, type, and devel- 

 opment of horn, etc. 



Meunier (1910) used girdle characters also in beginning the classi 

 fication of the genus, but considered first the character of the girdle, 

 whether flat and winged or impressed into the body of the organism 

 and not winged. 



The wing or hyaline list is, however, too superficial a structure to 

 claim far-reaching significance. Meunier 's system of classification 

 may be presented as follows : 



A. Planozones, girdle flat or but slightly, if at all, impressed, 

 winged. 



I. Girdle sinistro-spiral, e.g., P. divergens. 

 II. Girdle dextro-spiral (subdivided according to proportions 

 of major axes ; including the larger portion of the genus). 



B. Cavozones, girdle excavated, not winged, more or less clearly 

 sinistro-spiral, if not circular, e.g., P. thorimium, P. conicum. 



Meunier added also a short list of small forms which he finds it 

 difficult to insert elsewhere in his sj'stem. Examination of this system 

 reveals many misinterpretations of structure, if not errors in descrip- 

 tion, so that it cannot be retained as a useful contribution in revealing 

 the relationships of the genus. 



Plate Patterns. — Although descriptions of species had for many 

 years previous often been based upon the characters of plate pattern 

 as reported in sketches, together with the features of size, shape, 

 surface markings, etc., it was not until 1912 that a comprehensive sj^s- 

 tem for the whole genus was proposed on the basis of plate relation- 

 ships. Joergensen (1912) recognized on the ventral surface of the 

 epitheca two main groupings of the plates in this genus, those given 

 in our figures 3 and 4a, but he mentions also a third grouping which, 

 however, he regards as unimportant because of its infrequent occur- 

 rence, and probably only an exceptional occurrence of the pattern 

 of figure 1. He divides the genus primarily into two major sub- 

 divisions, giving the name, Orthoperidinium, to those species having 

 the plate pattern of figure 4a, and Metaperidinivni to those species 

 having the plate pattern of figure 3. To the remaining occasionally 



