1918] Barrows: Skeletal Variations in the Genus Peridinium 455 



From the absence of intervening stages showing the transition 

 from one pattern to the other ; from the fact that in the exceptional 

 patterns found in Mangin's figures, these patterns do not at all ap- 

 proach a transitional condition ; from the fact that the critical suture 

 of this unusual pattern is very long ; and from the position of the 

 mode of variation of length of the critical suture in the typical 

 divergens pattern with a minimum extreme for the curve for the 

 length of this suture which does not reach closer than .03 of the trans- 

 diameter to the vanishing point for this suture, we conclude that the 

 normal shape for plate 1" in this species carries a truncated anterior 

 portion where it meets plate 4' ; that with a truncation of its anterior 

 border of a greater or less extent, as represented by the curve of 

 variation given above, the plate pattern in this region is in a state 

 of equilibrium; that any force tending to upset this equilibrium 

 throws the arrangement of plates at once over into the only other 

 pattern possible, with already a critical suture of considerable length ; 

 and that a condition of two opposite plates in a pattern meeting at a 

 point without a truncated margin, and bringing together four suture 

 lines at one point is an extremely unstable and an unknown condition. 



In this connection a discussion of the suture relations of the dorsal 

 pattern of such a species as P. conicum is apropos because here the 

 critical sutures of the pattern, in this case that of fig. 6, are very 

 short. In fact, the pattern in this region has frequently been figured 

 as the intersection of four sutures. In a considerable number of speci- 

 mens of this species which we have examined, however, it has always 

 been possible to resolve these obscure relationships into the pattern 

 shown in fig. 6. In no case was found the alternate pattern that is 

 possible in this region. We regard this circumstance in P. conicum as 

 no different from that observable more clearly in P. divergens and in 

 other species : that a certain juxtaposition of plates in a given region 

 effects a condition of stability in the arrangement of these plates with 

 a certain amount of fluctuating variation in the length of the articula- 

 tions of the plates of the pattern ; that in a transition from one pat- 

 tern to the alternate a stage of instability is encountered of which 

 no representatives persist ; and that a more or less broadly truncated 

 juxtaposition of opposite plates of the quartette involved in a pattern 

 is essential to the establishment of equilibrium. This juxtaposition 

 manifests itself in presence of the so-called "critical suture," for 

 which a mode and extreme limits of variation can be determined. 



This method of analysis might be applied to all of the other cases 



