410 University of California Puhlications in Zoology [Vol. 18 



consider the type of horn as a primary basis in dividing the genus 

 into its general divisions. Two different types of horns arising from 

 the antapical plates are found in the genus, each represented in a 

 well-developed fashion in a large number of species. 



One of these tj'pes of antapical horns is formed by pushing out the 

 center of each antapical plate ot a greater or less extent, thus forming 

 a hollow horn which often becomes a large and prominent feature at 

 either side of the posterior end of the longitudinal groove which ter- 

 minates between the two horns. A spiny tip is often present at the 

 end of this hollow horn. The lists of the longitudinal groove which 

 are continuous with the posterior list of the girdle pass over the imier 

 bases of these horns and unite around the end of the longitudinal 

 groove. Occasionally, as in P. cummin gii Lemm, a fresh water form 

 having four horns, the hypotheca may be pushed out to form sharp 

 but hollow horns on other plates than the antapicals, or a general 

 elevation of the center of each antapical plate may terminate in two 

 or three sharp tips, as in P. crassipes Kofoid. 



The second type of horn seems to be quite unrelated to the former 

 types and consists of a solid spine, buttressed by three hyaline vanes, 

 two of which are continuous, except perhaps for a deep notch in the 

 vanes at the base of the spine, with the lists of the longitudinal groove. 

 The third vane lies on the opposite side of the spine from the longi- 

 tudinal groove. The spines are usually located upon the antapical 

 plates, but stand rather nearer to the longitudinal groove than do 

 the hollow horns. This type of horn reaches its greatest development 

 in such examples as P. steinii Jocrg. and P. tenuissimum Kofoid. 



It seems probable that this latter type of spine arose directly as a 

 support for the list of the longitudinal groove and thus independently 

 from the hollow type of horn and for a different purpose. In such a 

 species as P. crassipes, in fact, the incompletely developed stages of 

 both types of horn may be seen, the spiny type supporting the list of 

 the longitudinal groove and the hollow type represented by two or 

 three jDoints arising close together from near the center of each ant- 

 apical plate and producing the peculiar "flat-footed" appearance 

 for which the species was named. 



While in most species of Peridinium one type or the other of horn 

 is distinctly in evidence, there are species in which not only is there 

 no sign of elevation of the antapical plates to form the hollow type 

 of horn nor are the lists of the longitudinal groove high enough to 

 need support and even the spiny type of horn is indiscernible. This 



