GEODIA VARIOSPICULOSA. 59 



the ciioanosome these asters are very numerous; in parts of the interior they 

 are scarce. The oxysphaerasters are very numerous in the walls of the cortical 

 canals, zones exceedingly rich in them indicating the position of these canals 

 (Plate 18, figs. 21b, 23a, 26b). In the walls of the dermal canals these oxy- 

 sphaerasters extend right up to within a short distance of the surface, and they 

 are also met with in the walls, chiefly the roofs, of the subcortical cavities. A 

 few are scattered between the sterrasters. The small strongylosphaerasters 

 form a dense layer on the outer surface which increases in thickness in the pits 

 where the dermal membrane itself is thickened, and are also scattered through- 

 out the cortex. The sterrastei-s occur in the sterraster-armour, and are also 

 scattered in the choanosome. In all parts of the sterraster-armour layer, with 

 the exception of the thin portions of it in var. micraster, the sterrasters are rather 

 densely packed, in the thin parts of the cortex of the var. micraster they are 

 farther ajiart. The sterrasters in the choanosome are mostly young forms. 



The large choanosomal amphioxes (Plate 17, fig. 42) are usually curved, 

 often in an irregular wavy manner, gratlually attenuated to the rather sharj> 

 liointed ends, and isoactine or — as the one represented in the figure — slightly 

 anisoactine. In var. micraster they are 2.3-3.9 mm. long and 25-42 /x, usually 

 32-37 ,a thick; in var. intermedia considerably stouter, 2.5-3.1 mm. long and 

 42-50 ji, usually 42-44 ii thick. 



The large choanosomal tylostyles and styles (Plate 17, figs. 33, 43) are usually 

 curved. One of var. micraster which was intact measured 1.35 mm. in length. 

 These spicules gradually increase in thickness towards the rounded or tyle end. 

 Just below this they are in var. micraster 40-50 fi, in var. intermedia 25-35 fi 

 thick. Only a few of these spicules are true styles, in most the rounded end is 

 thickened more or less, sometimes so much so that the tyle is twice the 

 diameter of the shaft. But however great this thickening may be, it is never 

 sharply defined and passes gradually into the shaft, so that these spicules 

 appear more or less club shaped. The thickened end (tyle) measures in var. 

 micraster 46-62 fi and in var. intermedia 30-70 /< in diameter. 



The small dermal styles (Plate 19, figs. 4, 5) are fairly straight or slightly 

 curved, nearly cylindrical in the central part, and gradually attenuated towards 

 both ends. Distally these spicules nearly always terminate in a sharp point. 

 Very rarely the distal end is rounded and blunt. The thickness of the proximal 

 end, which is always rounded off, is from 25-67 % of the maximum thickness. 

 The small styles with a particularly thin proximal, rounded end are amphiox- 

 like. Many of these spicules are shghtly thickened locally at a point nearer 



