72 ECHINODERMATA. OPHIUROIDEA. 



into the mouth, giving it a stellate form. One of the 

 buccal plates serves as the madreporic plate. The genital 

 openings (g) also occur on the ventral surface of the disc, 

 they are in the form of slits in the interbrachial areas, 

 usually two in each, situated near the bases of the arms ; 

 occasionally there are four. The dorsal surface of the 



V 



FIG. 18. A, Ophioderma longicauda, Eecent. Ventral surface of disc 

 and part of the arms, b, buccal plates ; g, genital slits ; v, ventral 

 plates of arms. B, Ophioglypha, Eecent. Dorsal surface, r, radial 

 plates ; I, lateral plates of arms ; d, dorsal plates of arms. 



disc (fig. 17 B) is covered with scales or granules; and 

 usually there is at the bases of the arms on each side, a 

 large plate, the radial plate (r); the arms end abruptly 

 at the margin of the disc. 



Ophioderma (fig. 18 A). Arms long, round, smooth ; 

 the lateral plates provided with short spines or papillae. 

 Four genital slits in each area, two near the border of the 

 disc, two near the mouth arranged radially. Buccal plates 

 undivided. Dorsal surface of the disc finely granulated. 

 Lias to present day. 



