GEODIA ATAXASTRA. 



85 



ordinary amphiox forms with both ends sharp pointed. In var. angustana 

 these spicules are 120-200, mostly 160-200 p. long and 3-5 /( thick; in var. 

 latana somewhat larger, 145-215 /i long and 4-7 /z thick. The shorter ones are 

 on the whole thicker than the longer ones. In var. angustana the average 

 thickness of the minute rhabds less than 160 p. in length is 4.6 fi, that of the 

 ones more than 160 /t in length 3.9 /(. 



The orthoplagiotriaenes (Plate 43, figs. 9-22, 24a, 27a) have a straight, 

 usually conical and sharp-pointed, rarely shortened, cylindrical, and terminally 

 rounded rhabdome. The ordinary pointed rhabdomes are, in var. angustana, 

 1.6-2.3, mostly 1.9-2.2 mm. long and, at the cladome, 29-70, mostly 40-60 jx 

 thick, in var. latana they are considerably smaller, 1.3-1.7, mostly 1.5-1.6 mm. 

 long and 30-45, mostly 30-40 fi thick. The cylindrical, terminally rounded 

 rhabdomes are as thick as the conical ones at the cladome but only 330 /x-1.4 

 mm. long. 



I have studied the frequency of the different lengths of the ordinary pointed 

 orthoplagiotriaene-rhabdomes in a similar manner to the amphiox-lengths. 

 The curves representing the frequency of the difTerent lengths measured are 

 here reproduced (Fig. B). 



Per cents of all 



rhabdomes 



measured. 



So 



»>♦ 1^^ "^.^ ^?» >»» 



mm. in length. 



Fig. B. Pprcentag? frequency of lengths of rhabdomes of the orthoplagiotriaenes measured. 



Geodia ataxastra var. angustana. 



Geodia ataxastra var. latana. 



They are similar to the frequency-curves of the amphiox-lengths, and like 

 them exhibit a distinct main culmination, broad in var. latana and narrow in 



