144 



LIASSIC OPHIURID^. 



Disk round, small, flat, subpentagonal, almost circular ; arms long, smooth, cylin- 

 drical, tapering to a filiform termination ; vertebral pieces trilobed above. 



Dimensions. — Disk, half an inch in diameter ; arms, three inches in length. 



Description. — This is the most common of our fossil Ophiuridse, and many fine speci- 

 mens are obtained from the surface of the large blocks of micaceous sandstone vs^hich have 

 fallen from the Star-fish beds of the Middle Lias at Down Cliffs, between Charmouth and 

 Bridport Harbour, on the coast of Dorset. This species resembles Opldura texturaia, 

 Lamk., of our present seas, but differs in several important characters from that form. 



Some weathered specimens exhibit the structure of the rays in a most beautiful 

 manner ; others are so closely invested with the matrix, or covered with a ferruginous 

 crust, that it is impossible to make out their organic details. 



The disk is round, and slightly flattened at the interbrachial spaces ; its dorsal 

 surface is flat and smooth ; the radial plates are large, the pairs closely united by a 

 median suture, and to the adjoining plates by lateral sutures, so that the upper surface 

 of the body appears to be formed entirely by them. The ventral sm-face exhibits a buccal 

 opening with five rays (fig. 4, b) ; from the base of each arm two long narrow osselets 

 project inwards towards the mouth (fig. 4, b). The arms, six times as long as the 

 diameter of the disk, are slender, cylindrical, and taper gradually to the apex ; they 

 have on their dorsal surface a series of transverse scales, which, at the base of the arms, 

 are nearly as broad as they are long, in the middle twice as long as they are broad, 

 and proportionately more so towards the extremity ; the lateral scales closely embrace the 

 arm, and the spines, if present at all, must have been very short, as I have failed to 

 observe any traces of lateral spines in several well-preserved specimens. The basal scales 

 are nearly the counterpart of the dorsals as to form and structure. (PI. XV, fig. 5.) 



Affinities and Differences. — This species in its general form must have approached 

 very near Opldura texturata ; it differed, however, in the size of the radial plates and 

 structure of the disk. It closely resembles OpJdoderma tenuibracldata from the same 

 bed, "but in this species the rays are much longer in proportion, and less tapering; 

 they have a more flexible aspect than those of 0. Egertoni, and present in their section 

 a different form of the central ossicula ; for these, instead of being trilobate, are oblong, 

 with a triangular central anterior lobe." — Forbes, ' Proc. Geo. Soc.,' vol. iv. 



OpJdoderma Egertoni differs from 0. Gaveyi in having a smaller disk, with much more 

 slender arms, and still more from 0. Miller i, which is proportionately more robust than 

 0. Gaveyi. 



