REPORT ON THE OPHIUROIDEA. 255 



The skeleton of a Gorgonocephalus does not differ more from that of an ordinary 

 Ophiuran than those of Ophiurans differ among themselves. All the mouth parts are 

 present (figs. 18, 19) ; mouth frames (/), jaws (c), jaw plate (e), sockets for two sets of 

 tentacles (r,r f ), and a large peristomial plate (?>) in two pieces. There is, in addition, a 

 small angle cover (v'), which is strongly developed in most Astrophytons, and which 

 Ludwig considers the first under arm plate. The radial shield, genital plate (o), and 

 genital scale (fig. 17, n) occupy normal positions. In regard to the radial shields, this 

 peculiarity is to be remarked, that they are made up of a series of plates soldered one 

 on the other like tiles. This structure calls attention to the fact that radial shields, 

 which, from their almost constant presence, and their articulation with the genital plate, 

 are usually considered exceptional parts, are truly nothing more than a disk scale, or a 

 series of soldered disk scales. Hinged to the genital plates they regulate the position of 

 the roof of the disk as it is raised or lowered. Moreover, the genital plates themselves, 

 with their genital scales, are nothing more than highly specialised scales of the lower 

 interbrachial space, folded in, and bounding the genital openings on either side. In some 

 genera {e.g., Ophiomusium) the genital plate is externally conspicuous as one of the chief 

 pieces of the lower interbrachial space. The arm bones do not essentially differ from 

 those of Ophiurans, except that their joints are simplified so as to be adapted to rolling 

 in a vertical plane. The outer face has the usual transverse hour-glass projection, which 

 is vertical on the inner face. 



Passing now to Euryale, striking variations present themselves. The proportions 

 and arrangement of the mouth differ much from those of Gorgonocephalus. Two large, 

 flattened jaws (PL XXXV. fig. 1, c) support a small jaw plate (e), which carries, not 

 the usual bunch of spines, but a vertical row of flat teeth (d") like those of Ophiurans, or 

 those of Astroschema. Seen from above, the jaws (c) and mouth frames (f) are much 

 more solid than in Gorgonocephalus, and the peristomial plate (v), instead of being flat 

 and divided, is much swollen and single. While Gorgonocephalus has the under arm 

 plates in three pieces at the tip of the arm, and existing at its base only as irregular, 

 broken scales, Euryale has them nearly or quite unbroken (figs. 1, 6, h) and of a 

 regular form for the whole length of the arm except the terminal twigs. It is at the end 

 of the arm that there is a remarkable difference in the side plates of the two genera. 

 Gorgonocephalus has small thick plates clinging close to the arm, whde Euryale is 

 furnished with long, finger-like projections standing free and bearing prehensile hooks 

 (figs. 8-12). 



A side arm plate of the same general character may be found at the tip of the arm of 

 OpJiiothrix pusilla. 1 Passing towards the disk, these plates grow shorter and wider, and 

 then hooks thicken into club spines (figs. 8, 9, i,g). Still further inward the side plates are 

 nearly like those of Ophiurans, and carry little conical tentacle scales (or arm spines) on 



1 Bull. Mus. Comp. Zool., vol. iii. p. 10, pi. iii. fig. 28. 



