ERYLUS SOLLASII. 

 DIMENSIONS OF ACANTHTYLASTERS. 



283 



The adult aspidasters of Erylus sollasii (Plate 1, fig. 36d; Plate 2, figs. 5-7, 

 12-15, 19-26; Plate 3, figs. 9-11, 16, 17, 25d, 28) are 95-156 /.i long, 55-82// 

 broad, and 74-14 /( thick. The general average proportion of their length to 

 their l^readth to their thickness is 100:55.8:8.8. Optical transverse sections 

 show that these disc-shaped spicules are gradually attenuated towards the mar- 

 gin, which is usually quite sharp. The shape of their outline is variable; some 

 of them (Plate 2, figs. 13, 23, 26; Plate 3, fig. 9) are quite regularly oval, some 

 rounded rhomboidal (Plate 2, fig. 22; Plate 3, figs. 10, 17), and some irregular 

 (Plate 2, figs. 12, 24, 25) with lobose marginal protuberances of which one or 

 a few broad ones, or a larger number of narrow ones may be present. On one 

 face of the disc there is a very shallow, more or less circular depression 20-30 fi 

 in diameter. In this depression, which is obviously homologous to the umbili- 

 cus of the sterrasters of Geodia, the surface is either quite smooth, or bears only 

 a few small rays or spines. From all the other parts of the surface (Plate 2, 

 figs. 5, 6; Plate 3, fig. 28) and also from the margin, rays usually about 1-2 // 

 thick protrude. Those on the margin are about 1.5 /.i long, those on the faces 

 appear to be shorter. These rays are scattered rather irregularly and (meas- 

 ured from centre to centre) 2-6 /x apart. Each ray bears a terminal verticil 

 of four to ten lateral spines. The centre of the aspidaster is occupied by a 

 small group of granules, from which very numerous and perfectly straight radial 



