1918] Barrows: Skeletal Variations in the Genus Peridinium 451 



ventral aspect, which is foreign to the usual form of the species. 

 Applying this analysis still further to the case of P. divergens, this 

 circumstance in the first place demonstrates the plasticity of this region 

 on the right side of the ventral surface of the epitheca. However, 

 a greater significance appears from examining the lengths of the 

 critical sutures in this region of the shell of P. divergens. In no case 

 does the value of the suture between plates 4' and 7" fall below .04 

 of the transdiameter in the pattern normal for the species. That 

 is, these two plates always meet each other for a definite length of 

 their periphery. The suture separating them always maintains a cer- 

 tain definite and considerable value. The widening of sutures in 

 certain specimens, presumably with age, may mask this appearance, 

 though it is possible to find that the opposed margins of these two 

 plates always present edges of definite and considerable length and 

 that the plates never really meet by two points bringing together the 

 four sutures of their adjacent edges at one intersection point. Nor 

 in any other instance have more than three sutures been found to 

 come together at one point. Instances of the apparent meeting of 

 four sutures at one point upon critical examination show an inter- 

 vening length of suture between two intersection points of three 

 sutures each; never do four plates actually come into contact at one 

 point. Here, then, is the record of forty-four random instances in 

 which this suture between plates 4' and 7" varying through a consider- 

 able range never falls below .03 of the transdiameter. 



In contrast to this usual condition, a few instances are known in 

 which an alternate pattern has appeared, the only other pattern pos- 

 sible by any rearrangement of plates at this point. This alternate 

 pattern is caused by the failure of plate 7'' to meet plate 4' as usual. 

 In this alternate and unusual pattern it is, then, the suture between 

 plates V and 6" which is of critical significance. This suture in the 

 two exceptional cases figured by Mangin (1911, pi. 7, figs. 10 and 13) 

 is of considerable length and is not short as though the usual pattern 

 for the genus had barely slipped over into the alternate pattern. In 

 two other cases observed by the writer, but unfortunately not recorded 

 by drawings, this suture between plates V and 6" was also of con- 

 siderable value. 



Turning now to the varying length of the critical sutures in the 

 three other regions of varying plate pattern in this species, P. diver- 

 gens, there is not only no suggestion of a merging of the given pattern 

 for P. divergens with the alternate pattern possible in any of these 



