448 University of California Puhlications in Zoology [Vol. 18 



Variations in the Length of Critical Sutures 



Further light upon the manner of change of plate pattern may 

 come from an examination of the varying length of the sutures in 

 these variable areas, particularly of those sutures which drop out and 

 of those new ones which are formed in the transposition from one 

 plate pattern to the next. It will be recalled that the dorsal plate 

 pattern of P. divergens Ehrbg. is that of fig. 5. The only known 

 symmetrical change in this pattern involves the obliteration of the 

 suture lines between plates la and 4" and plates 3a and 4" by the 

 enlargement of plate 2a so as to establish articulation expressed by 

 the sutures between plates 2a and 3" and plates 2a and 5". In a long 

 series of personal observations and from an examination of published 

 figures of P. divergens, the writer is not aware that this typical dorsal 

 plate pattern for P. divergens is ever altered in this species. Measure- 

 ments of these critical sutures, between plates la and 4" and plates 3a 

 and 4" have been made for a total of forty-four specimens taken at 

 random in groups of from six to eleven specimens from each of six 

 regions : Yes Bay, Alaska ; two localities in the vicinity of San Diego, 

 California; from Christineberg in the Skagerak; from Cette on the 

 Mediterranean; and from Naples. The results of these measure- 

 ments are expressed in the accompanying chart (fig. 17), which shows 

 that the range of variation for the length of this suture is from .0405 

 to .1810 of the transdiameter with a double crested norm with maxima 

 at .07 and .09. It is to be noted that in no one of these instances 

 measured did the length of this suture fall below .04 of the transdiam- 

 eter of the specimen. 



The length of the corresponding suture between plates 3a and 4" 

 passes through a somewhat similar range of variation from .0405 to 

 .2134 of the transdiameter with a double crested norm and maxima at 

 .06 and .08 and with no measurement less than .04 of the trans- 

 diameter. 



The critical sutures of the variable areas of the ventral side of 

 the epitheca have also been measured for most of the same specimens 

 and for a few others, totaling again forty-four specimens, with the 

 result that the length of the suture between plates 4' and 1" has been 

 found to range in this series from .027 to .1101 and in one instance 

 to .1397 of the transdiameter, with again a double crested norm with 

 maxima at .05 and .08 (fig. 18). 



