GEODIA AGASSIZII. 



125 



the rounded part is in favor of the view that the former is the rhabdome and the 

 latter the epirhabd. Since, however, the rounded part is the thicker of the two, 

 since the clades have their concave side turned towards this shorter and thicker 

 part; and since there can be httle doubt that these spicules are derivates of the 

 orthoplagiotriaenes, in which the concave side of the clades is invariably turned 

 towards the rhabdome, it seems that the short, thick, and rounded part of the 

 shaft should be considered as the rhabdome and the long, thin, and pointed part 

 as the epirhabd. 



The afnphiclade orthoplagiotriaene-derivates (Plate 26, fig. 2; Plate 29, 

 fig. 13; Plate 34, fig. 13) are also very rare. They have been found only in the 

 adult specimens from Stations 3168 and 4199 and in the young specimen from 

 Station 4228. They differ from the orthoplagiotriaenes described above only 

 by possessing, besides the terminal cladome proper, a short, rounded or pointed 

 clade about 100 150 /t long, which arises at the acladomal end of the rhabdome 

 or some other part of it more or less remote from the true cladome. 



Besides these orthoplagiotriaene-derivates a few quite irregular spicules 

 have been observed, which, to judge from their general character, appear to be 

 derivates either of the orthoplagiotriaenes or of the large choanosomal amphi- 

 oxes. On Plate 28 photographs of some of these spicules are reproduced. One, 

 fig. 8, is a triaene with a shaft 35 p. thick at the cladomal end, and three straight, 

 conical clades, 130 /( long, approximately extending in a plane which passes 

 through the rhabdome. One, fig. 10, is a stout, large amphiox with two straight 

 and pointed, clade-like branch-rays, 165 /t long, arising 250 /< below one of the ends 

 and extending very obliquely downward towards the centre of the amphiox. 

 One, fig. 9, has the appearance of a large amphiox, one end of which is replaced 

 by a centrally attached, obliquely situated, style-like rhabd, 430 /jl long. One, 

 fig. 11, is a large amphiox, from which, at a distance of 260 fi from one of the 

 ends, a straight, conical, clade-like branch, 240 fi long, arises vertically. 



Of other irregular spicules observed I mention an amphistrongyle, about 

 1 mm. long and thicker at one end than the other, with a straight, conical, branch- 

 ray 80 n long, arising obliquely 70 // below the thinner end and directed towards 

 the thicker end. 



The mesoprotriaenes (Plate 28, figs. 1-7, 16d; Plate 32, figs. 40, 41) occur in 

 all specimens. Their rhabdome, which is thicker in the middle than at either 

 end, is straight or only slightly curved, and in the adult specimens 2-6 mm. long 

 and at the cladome 7-40 thick, the average maximum dimensions being 5.1 

 mm. X 25.11 /.(. The rhabdomes of the mesoprotriaenes of the young specimen 



