R. ETHERIDGE, JTTN., ON ASTROCRINTTES. 107 



crossed by vertical scale-like ridges (fig. 13) instead of being orna- 

 mented with tubercles. It has a projecting lip (/, fig. 5 ; e, fig. 20), 

 with a minute inward ridge (g, fig. 5). Following this plate round 

 towards the dorsal surface, we find that its pointed extremity 

 (a, figs. 13 & 14 ; /, fig. 20) is received into the reentering angle of 

 yet another plate, which may be seen in the end view of a specimen 

 represented in fig. 15 (a), passing on to the dorsal surface, with 

 which it unites. Whether this (a, fig. 15) is a single plate or com- 

 posed of two portions, I am in doubt. In some individuals it ap- 

 pears to be entire (6, fig. 12; d, fig. 11), whilst in others there is 

 certainly a division down the centre from the reentering angle 

 separating it into two halves (a, a', figs. 15 & 16). In fig. 16 we 

 have represented an internal view of the crushed hollow dorsal surface 

 (with, at b, such a separation of the plate in question into two halves), 

 the only remaining portion of the unsymmetrical lobe. The aper- 

 ture-plate (e, fig. 5 ; a, fig. 15 ; d, fig. 20) presents, when viewed 

 internally, some points of interest. The inward projecting ridge 

 described as at g (fig. 5) is seen to be a portion of a minute circular 

 cup-like depression (b, fig. 14), which leads inwards and downwards 

 by a small channel or groove (c, fig. 14) towards the general cavity 

 of the body. At the edge of the interior and vertical portion of the 

 plate this channel appears to break up into two or three folds 

 (d, fig. 14). 



1 have endeavoured in the foregoing description, with the aid of 

 Mr. Sharman's truthful figures, to convey some idea of the plates 

 composing this little organism, although it is difficult to convey to 

 others a clear and comprehensive view of its structure, especially 

 when we take into consideration its small size and usually crushed 

 condition. 



(c) Pseudambulacra. — The pseudambulacra are four in number 

 (a, fig. 1), long, narrow, very slightly petaloid, and radiate outwards 

 from the central aperture, one to each of the four concavities or inter- 

 lobular spaces between the arched and convex lobes of the calyx. Each 

 pseudambulacrum is more or less hexagonal in section, as indicated in 

 fig. 18, which is a diagrammatic cross section of one Mr. Bennie has 

 succeeded in obtaining separated from the remainder of the organism 

 (fig. 19). The component parts of the pseudambulacra are usually so 

 closely fitted together that the plates can only be occasionally dis- 

 tinguished. The more or less nearly horizontal upper surface 

 is traversed by a median (and for the size of the organism), 

 deep longitudinal groove. The sides of each pseudambulacrum are 

 bent down at a considerable angle, and in weathered specimens 

 are seen to be formed by two series of small plates on each side. The 

 pseudambulacral groove (a, fig. 17) gives off other, short, lateral, alter- 

 nate grooves (b, fig. 17), which terminate in small and nearly round 

 depressions (e, fig. 17) or sockets. Contiguous to each of these, and 

 separated only by a small interspace, is another depression (d, fig. 17) 

 somewhat more than halfway between the psoudambulacral groove and 

 the margin, also connected by another groove (/, fig. 17) with a slit- 

 like opening (or pore ?) {e, fig. 17) between the side plates g and h. 



