158 



OOLITIC OPHIURID^. 



Genus — Amphiuua, Forbes, 1844. 



Discal body orbicular, having its upper surface covered with small, smooth plates, 

 the six central plates forming a rosette. The simple scaly rays arising from the 

 centre of the disk are provided with lateral subcariiiated plates, carrying from 5 — 12 

 simple lanceolate spines. Ambulacral papillaj either absent, single, or double. In 

 Aijqjhiura tenera the disk is small and not lobed, the short delicate arms are surrounded 

 with triradiate spines ; the radial plates are small, the ventral plates pentagonal, and 

 both are naked. 



Amphiura Prattii, Forbes. PI. XVIII, figs. 1 a — d, 2. 



Amphidua Prattii, Forbes. Proceed. Geol. Soc, vol. iv, p. 233, 1844. 



Disk with a few smooth imbricated scales on the intermediate plates beneath. 

 Arms in small specimen nearly six times as long as the diameter of the disk ; slender 

 and flexible, and not tapering much. The inferior ray-scales are quadrangular, 

 with oblique sides. Each lateral ray-plate bears a row of slender, conical, diverging, 

 smooth spines, which are about as long as the breadth of the ray, and there is also a very 

 small spine at the inferior angle of each. 



Dimensions. — Fig. 1. Diameter of the disk unknown ; length of the rays from the 

 peristome to the apex one inch and three tenths. Fig. 2. Disk small, rays six times as 

 long as the diameter of the body. 



Description. — This rare Ophiurid presents only the under surface of the animal, 

 the disk, which was probably soft or cartilaginous, having disappeared, though its limits 

 are well marked by the forms of the ossicula composing the bases of the arms which were 

 inserted into it. There are traces of a few smooth imbricated scales on the intermediate 

 plates beneath. The smaller specimen coiled up on a slab (fig. 2, natural size) shows how 

 small the discal body was in proportion to the length of the arms, which were at least six 

 times as long as the diameter of the disk. The under side of the arms is composed of a 

 central plate of hexagonal shape, with an ambulacral hole on each side (fig. 1 a, b, c, d). 

 The lateral plates are imbricated and their free border is provided with five or six stiff 

 thorn-like spines (fig. 1 b, c, d), in which the structure of the arm is satisfactorily shown. 

 The foramina between the central and lateral plates are well seen in these figures. The 

 small specimen (fig. 2) does not exhibit much of its structure, and we are at present 



