ERYLUS SOLLASII. 



283 



DIMENSIONS OF ACANTHTYLASTERS. 





Race 



all forms 



I 



II 



III 



Forms 



A 



B 



C 



D 



limits 



general average 

 of tlie largest 

 three 



Number of rays 



4-12 



3-13 



3-10 



3 14 



2-14 



4-12 



2-14 





Diameter 



of all asters 

 measured with 

 six rays or 

 fewer, /< 



20-36 



21-32 



21-32 



22-30 



22-3S 



25-36 



15-38 





of all asters 

 measured with 

 seven or more 

 rays, /< 



10-27 



6-20 



17-22 



12-27 



12-24 



13-24 



10-27 





average of the 

 three largest, /<. 



33 



29 



31 



29 



35 



33 





31.7 



The adult aspidasters of Erylus sullasii (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 /i thick. The general average proportion of their length to 

 their breadth 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 fi 

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

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

 ured from centre to centre) 2-6 /t 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 



