GEODIA ATAXASTRA. 



87 



the one hand and the clade-angle and the rhabdome-thickness (at the ckidomc) 

 on the other I measured these dimensions in eighty-one cladomes taken at 

 random. I arranged these measurements in the order of the ckade-Icngth, and 

 divided up the series at intervals of 10 /x, considering all clade-lengths from 145 to 

 155 as about 150 all from 155 to 165 as about 160 /< and so on. All the meas- 

 urements of clade-angles and rhabdome-thickness pertaining to clades of similar 

 length (150, 160, and so on) were combined and their averages taken. These 

 averages were then plotted on the ordinates erected in the points of the hori- 

 zontal axis representing the clade-length (150, 160 n, and so on). The curves 

 connecting the points thus plotted in are here reproduced. 



Averages of H, 

 thickness of rhabd- 

 onie of orthopla- 

 giotriaenes with 

 clades of that 

 length. 



, , ^ . 



/( clade-length. 



Fig. D. Correlation between the clade-length and rhabdome-thickness of the orthoplagiotriaenes of 

 Geodia ataxastra var. angustana. 



These curves are very irregular, but they show nevertheless that the clade- 

 angle on the whole decreases with increasing clade-length, while the rhabdome- 

 thickness increases with it. That is to say, that, roughly speaking, the width of 

 the clade-angle is in inverse, the thickness of the cladomal end of the rhabdome 

 in true proportion to the clade-length. According to the curves the clade-angle 

 and the thickness of the cladomal ends of the rhabdomes do not seem to be 

 correlated with each other any more closely than with the clade-length. On the 

 whole it may therefore be said, that, although there undoubtedly exists a corre- 



