40 HOLASCELLA ANCORATA. 



The svialler, spined hexactine and tetractine megascleres (Plate 24, figs. 1, 2) 

 are 1-4 mm. in maximum diameter. Their rays are unequal, often curved, 

 up to 1.7 mm. long, 12-17 ^ thick at the base, and rounded at the end or blunt- 

 pointed. The bases and often also the tips of the rays are smooth, the other 

 parts show sparse, broad, sharp-pointed spines. 



The comital spicules (Plate 24, fig. 9) are di- to tetractine. Their rays are 

 straight or irregularly curved, gradually attenuated distally, and terminally 

 rounded. The end-part is usually somewhat thickened and spined. The other 

 parts of the spicule are smooth. The rays attain a very considerable length. 

 Measurements of this dimension cannot, however, be given since all the long 

 rays observed were broken off. The longest intact ones seen were 1.5 mm. long. 

 The rays are 8-28 ^ thick at the base and attenuated distally to 5-8 tx. The 

 spined end-part is 7-10 m thick. In the tetractine and triactine comitals two 

 opposite longitudinal rays lie in a straight line and are longer than the transverse 

 ones (one) . In the triactine forms the centre is markedly thickened on the side 

 opposite the single transverse ray (Plate 24, fig. 9). The diactine forms are cen- 

 trotyle. The central tyle measures 14-36 m in diameter. The proportion of the 

 basal thickness of the rays to the transverse diameter of the tyle is 1 : 1.4 to 1 : 3, 

 usually about 1: 1.6. The two rays of these spicules are usually unequal in 

 length and sometimes one of them is reduced to a mere knob. Such excessive 

 longitudinal reduction is always associated with a considerable thickening. 

 In an extreme form of this kind one ray was observed to be over 2 mm., the 

 other only 44 m, long. The central tyle measures 40 ^ in diameter; the long ray 

 is 16 ju thick and nearly cylindrical. The short ray is 33 ^ thick at the base and 

 farther on it is 44 /x thick. This knob-like rudimentary ray is covered with small 

 spines down to within a short distance of its base. 



The proximal and lateral rays of the hypodermal and hypocjastral hexactines 

 (Plate 23, figs. 12, 13) are 5-9 m thick at the base. They are cylindrical or 

 slightly attenuated distally, and usually rounded at the end, rarely pointed. 

 Their tips are generally spined, their other parts smooth. The proximal ray 

 is 0.9-1.5 mm. long, the lateral rays 370-450 m- The distal ray is 220-450 ^ 

 long, at the base as thick or somewhat thinner than the other rays, and distally 

 thickened. At its thickest point, which lies near the distal end, it measures 

 17-40 n in diameter. The proximal part and the extreme tip are smooth, the 

 other parts of it more or less spined. The spines increase in size and number 

 distally. They arise very obliquely and point towards the tip of the ray. The 

 hypodermals are similar to the hypogastrals. Hexactines with thick strongly 



