60 



GEODIA VARIOSPICULOSA. 



the rounded than the pointed end. I presume that this thickening is situated 

 at the point where the spicule penetrates the surface and that it is formed after 

 the spicule has taken up its definite position in the low spicule-fur. In var. 

 ■m icraster these small dermal styles are 210-320 fi long, 3-7 fi thick in the centre, 

 and 1-3.5 /i thick at the proximal, rounded end. In var. intermedia they are 

 somewhat shorter, 200-310 i-i long, 3.5-7 a« thick in the centre, and 1-2.5 n thick 

 at the proximal, rounded end. 



A concrescent form of these spicules, consisting of three or four straight 

 rays, some pointed, others rounded at the end, is very rarely met with. 



The large orthoplagiotriaenes (Plate 17, figs. 34-37, 38a, 44-47, 49) have 

 generally a straight, or slightly curved, conical rhabdome, pointed at the 

 acladomal end, more or less thickened at a distance of about 150 /i below the 

 cladomal end, and considerably constricted above this thickest point, just below 

 the cladome. In the neck-like subcladomal constriction the thickness of the 

 rhabdome is from 65-85 % of the thickness of its thickest part. At this point, 

 weakened as it is by the constriction, the rhabdome readily breaks, and parts 

 of these spicules (cladomes and rhabdomes) broken at that point are frequently 

 found in the spicule-preparations. The chords of the clades form angles of 

 99°-lll° with the axis of the rhabdome. These spicules are consequently 

 intermediate between orthotriaenes and plagiotriaenes, and are named ortho- 

 plagiotriaenes accordingly. The clades are conical, not sharply pointed, fre- 

 quently quite blunt. They always arise in an ascending direction. Their 

 proximal part is concave to the rhabdome, their distal part straight (Plate 17, 

 figs. 34-37, 47) or slightly undulating (Plate 17, figs. 44, 45). A shght, abrupt, 

 angular bend is frequently observed at the point where the proximal part, 

 concave to the rhabdome, passes into the distal straight or undulating part. 

 This and a slight thickening of the axial thread of the clade often observed at 

 this point seem to indicate that these orthoplagiotriaenes are dichotriaene- 

 derivates. The three clades of the same cladome usually arise at similar angles 

 from the rhabdome; forms like the one represented, Plate 17, fig. 38a, in which 

 the rhabdome angles of the three clades differ to a greater extent, being rare. 

 In length the clades of the same cladome may be equal or unequal. The forms 

 with unequal clades are as numerous as the ones with equal clades, if not 

 more so. Most of the cladomes composed of unequal clades are sagittal,- two 

 clades being fairly equal, while the third is very much shorter, only one half or 

 a third as long, as the other two. The angles between the clades are independ- 

 ent of the clade-length and nearly always about 120°. Rather frequently a 



