406 University of California Publications in Zoology [Vol. 18 



it as a basic feature in composing a system of nomenclatiTre. From 

 this basis of reference the plates in Peridininm at once appear as 

 arranged in circumferential rows, one row on either side of the girdle, 

 the precingular row and the postcingular row ; one group, the apicals, 

 about the anterior pole or apex ; an incomplete row interpolated be- 

 tvy'een the apicals and the precingulars, the anterior accessory plates ; 

 and in this genus, a pair of plates over the posterior end, the antapi- 

 cals, carrying the antapical horns, and representing an antapical row. 



Each plate is designated by a composite symbol of which one 

 factor, the superscript written as a prime point indicates the row, the 

 rows themselves being designated by one, two, three, or four prime 

 points as they occur in succession from the apex posteriorly. The 

 incomplete row of anterior accessory plates is designated by the 

 superscript a. In each row the plates are numbered from the left to 

 the right of the organism coincidently with the direction of the course 

 of the transverse flagellum. This direction for numbering is, as one 

 looks at the organism from the anterior end, counter-clock-wise for 

 the epitheca and as seen from the posterior end, clock-wise for the 

 hypotheca. This order of numbering corresponds to the usual method 

 for numbering serial parts distally from the proximal portion. 



Thus, in Peridininm the apicals are numbered from 1' to 4', the 

 anterior accessories from a 1°, to 3", the precingulars from 1" to 7", 

 the postcingulars from 1'" to 5"', and the antapicals 1"" and 2"". 

 This system permits the writing of a simply expressed formida for 

 the plate relations, the number of girdle plates being inserted with- 

 out superscript or if desired for completeness with the superscript g 

 (which, however, has not been used by Kofoid). For a typical species 

 of Peridi7uuni the plate formula would then be 4'-3''-7"-3''-5'"-2"". 



This sj^stem presents much practical convenience. The numerals 

 as symbols more rapidl}^ indicate the relative position of a plate in its 

 row than do the letters used by Biitschli (1885, p. 928), Faure-Freimet 

 (1908, p. 215), Stein (1883, pi. 10, figs. 1, 6), and Broch (1912, fig. 8), 

 while the superscript indicates at once the relative position of the row 

 antero-posteriorly. Not the least favorable feature of Kofoid 's system 

 is its elasticity and adaptability to any member of the group of dino- 

 flagellates which has a skeleton made up of discreet plates. 



This s^-stem of nomenclature, however, is based on more funda- 

 mental grounds than those of mere convenience. The fact that this 

 system, regarding the plates as grouped in circumferential rows, holds 

 throughout a large portion of this extensive group is in itself signifi- 



