86 



GEODIA ATAXASTRA. 



var. angustana, which shows that the "normal" lengths of the orthoplagio- 

 triaene-rhabdomes are 1.5-1.6 mm. in var. latana and 2 mm. in var. angustana. 

 Also in these curves a minor culmination, preceding the main one, is observed, 

 and I think there can be little doubt that this indicates, as in the case of the 

 amphiox-lengths, a temporary cessation or a retardation of longitudinal growth 

 of the orthoplagiotriaene-rhabdomes, in this case when they are 1.3 mm. long in 

 var. latana and when they are 1.6 mm. long in var. angustana. In these 

 curves, as in the ones pertaining to the amphioxes, the part beyond the main 

 culmination descends very rapidly at first and less rapidly later on. This 

 seems to show that most of the amphioxes and orthoplagiotriaene-rhabdomes 

 becoming longer than the normal grow beyond this very rapidly. 



°, average of 

 angles of clades 

 of that length. 



^/•^■| I I T , 1 ^ , , 1 



9f 



<:^^^t^^S$• ^3:35 ij^^^SX^^ /< dade-length. 



Fig. C. Correlation between the lengths and angles of the clades of the orthoplagiotriaenes of Geodia 

 ataxastra var. angustana. 



The cladomes of most of the orthoplagiotriaenes are regular, the three 

 clades of the same cladome being nearly equal in length, position, and curvature. 

 The clades of these regular orthoplagiotriaene-cladomes are conical, pointed, 

 and curved, concave to the rhabdome, rather strongly in their proximal and 

 central parts but only slightly or not at all in their distal part. In var. angu- 

 stana they are 170-290, mostly 230-270 long, in var. latana shorter, only 130- 

 220 fj. long. The angle enclosed between the clade-chords and the rhabdome- 

 axis is in var. angustana 85-116°, on an average 100.3°, in var. latana 95-111°, 

 on an average 101°. Considering, as I do, teloclades with a clade-angle between 

 80° and 100° as orthoclades and teloclades with a clade-angle of 100-120° as 

 plagioclades, I name these spicules orthoplagiotriaenes. 



For the purpose of ascertaining the correlation between the clade-length on 



