450 University of California Publications in Zoology [Vol. 18 



us^^ally contiguous — an instability of so great a moment as to produce 

 a plate pattern very different from that characteristic for the species 

 in this region. This instability may, moreover, be located largely in a 

 single plate, 7", and the inference goes to support the previous sug- 

 gestion that plate 7'' in Peridinium is of late phylogenetic origin. 



In the two specimens of P. divergens figured by Mangin (1911, 

 pi. 7, figs. 10 and 13), which have an unusual pattern, the critical 

 sutures are well developed and the pattern is unmistakable. Measure- 

 ments of the unusual sutures between plates 1' and 6" give values of 

 .1071 and .1587 of the transdiameter, and measurements of the cor- 

 responding suture on the left hand of the rhomboid plate, which is of 

 the normal pattern for this species, give values of .0357 and .1270 

 of the transverse diameter. 



The unusually short length of the suture between plates V and 2" 

 in one of Mangin 's figures is noteworthy in view of the usual great 

 length of this suture in most of the specimens of this species measured, 

 and is suggestive of the profound effect of any influence which would 

 upset the normal equilibrium of the morphological elements. 



These measurements were taken from drawings made to a scale of 

 1 :1500, usually from square ventral or dorsal views. There is, how- 

 ever, a certain amount of foreshortening in these drawings of sutures 

 which lie upon curved surfaces that is unavoidable. Instead of mak- 

 ing an allowance for this foreshortening the sutures have been mea- 

 sured just as they have appeared in the drawings. The drawings, 

 made with camera lucida, are thus projections of the surface of the 

 organisms which they portray. The general effect upon these measure- 

 ments is that many of the values given are less than the actual length 

 of the sutures represented. In no case is an exaggeration of measure- 

 ment possible. Any errors from this method of measuring the suture 

 are, therefore, in favor of the general conclusions which will presently 

 be drawn in examining the significance of the measurements. 



Significance of Variation in Suture Length 



The occurrence of a few specimens of P. divergens with an unusual 

 ventral plate pattern is to be regarded as of very great importance. 

 Of equal importance in this consideration is the occurrence of species 

 answering the descriptions of P. ovatum, P. claudicans, P. oceanicimi, 

 and P. granii in every respect except that of the plate pattern, and 

 in this one character presenting plate patterns on either dorsal or 



