HYALOx\EMA (HYALOXEMA) OBTUSU.M. 165 



In war. robusta they are larger, sometimes 1.1 mm. long and 20 ^ thick. A slight 

 central thickening (tyle) with an axial cross in the interior can usually be made 

 out, particularly in the smaller uncinates. This is generally about 0.5, rarely 

 as much as 1 yn thicker than the adjacent parts of the spicule. The spicule is 

 covered with slender spines, all strongly inclined in the same direction. Near 

 the end from which these spines diverge, they are rather numerous, towards the 

 other end they become very scarce. So far as I could make out these spines 

 consist of a rather broad conic basal part and a fine, exceedingly slender, needle- 

 like end-part. The basal conic part arises steeply from the shaft and bends 

 round above, where it passes into the fine end-part, so that the latter comes to 

 lie nearly parallel to the shaft. 



The large stalk-spicules (Plate 40, figs. 21, 22), in var. roMista 8 mm. below 

 their upper ends, where they are all broken off, are 40-720 ix thick. The empty 

 spaces previously occupied by them in var. gracilis have a maximum width of 

 900 fi. The upper ends of these spicules of var. robusta are curved, the curvature 

 increasing towards the (upper) end. The axial thread is for the most part 3-4 ^ 

 thick. It does not lie centrally, but describes a spiral line around the mathe- 

 matical axis of the nearly cylindrical spicule. It is by no means a simple cylindri- 

 cal thread. Some parts of it (Plate 40, fig. 21) are uncinate-like, covered with 

 strongly inclined spine-like processes directed upwards, others (Plate 40, fig. 22) 

 are thickened, quite irregular, and attain 20 fi in transverse diameter. 



In both varieties the regular microhexactines (Plate 35, figs. 14, 15, 17a, 18, 

 19; Plate 40, figs. 6, 7, 20b) measure 42-80 // in diameter. The six rays of the 

 same spicule are fairly equal, and regularly arranged. The chords of the rays 

 are 20^3 m long. The rays themselves are 1.5-2.2 ^ thick at the base, gradually 

 and uniformly attenuated distally to a fine point, and covered with very minute, 

 vertically arising spines. The basal parts of the rays are nearly straight, the 

 distal parts strongly curved through an angle usually a little over 90°. The 

 direction of curvature of the end-part of each individual ray is generally opposite 

 to that of the end-part of the ray opposite it in the same axis. 



The microhexactine-derivates (Plate 35, figs. 20-22; Plate 40, figs. 8-15, 20c) 

 represent two series of forms. One begins with microhexactines in which the 

 two rays lying opposite in the same axis are longer than the other four, and ends 

 with centrotyle diactines. The other begins with micropentactines with equal 

 rays, and ends with style monactines. In var. robusta forms of both series are 

 rather frequent ; in var. gracilis hardly any but diactine forms, with the two fully 

 developed rays opposite in the same axis, have been observed. 



First series of microhexactine-derivates. One of the microhexactines, with 



