136 OPHIUROIDEA. 



Opldothrix has the orbicular or pentagonal disk covered with very fine spines, more 

 or less developed ; sometimes they are so delicate that the clothing has quite a villous 

 character ; the radial plates are divergent and naked, or partially covered near their base 

 with the general clothing of the disk. 0}ildothrix Mammehhergii (PI. XIII, fig. 8) belongs 

 to the group in which the disk is provided with small short cylinders ; the sides form a 

 prominence between the arms ; the radial plates are divergent and naked, except near 

 the base, where they are provided with small granulations. 



Protaster, an extinct genus, has the circular disk covered with squamiform plates ; 

 the genital openings are in the angles of junction of the rays beneath, and the arms are 

 formed of alternating ossicula. Plate XIV, fig. 9, exhibits these characters as shown in 

 Frotasfer Sed(/wickii, from the Ludlow rocks of the Upper Silmian series. 



Ampldura has an orbicular disk, having its upper sm-face covered with small smooth 

 scales, the six central plates forming a rosette. The arms, simple and scaly, arise from 

 the centre of the disk, and are provided with lateral subcarinated plates carrying simple 

 lanceolate spines. In Ampldura tenera (Plate XIV, fig. 10, a, b) the disc is small and 

 not lobed, the short delicate arms are sm-rounded with tri-radiate spines ; the radial 

 plates are small, the ventral plates pentagonal, and both are naked. 



Aspidura, an extinct form from the Muschelkalk of Germany (PL XIV, fig. 6, a, b) 

 has the upper surface of the body covered with fifteen plates ; ten of these form the outer 

 and five the inner circle of the small disk ; the arms are furnished with four rows of 

 plates of unequal sizes. 



In most genera the upper surface of the disk supports at the base of the arms two 

 large calcareous pieces called radial plates, which are sometimes placed close together, as in 

 OpJdura texturata, or apart, as in Ophiolepis annulosa (PI. XIII, fig. 2). They are either 

 entirely naked, as in Ophiolepis and Opldarachna, or partly clothed with the general 

 covering of the disk, as in Opldothrix. The radial plates are well preserved in situ 

 in many fossil species, as Ophioderma Milleri and 0. Gaveyi (PI. XVI and XVII). 



On the under surface of the disk are five interbrachial spaces between the arms 

 (Plate XIII, figs. 1, 3, 4, 5, 6). In each of these are large smooth pieces, called buccal plates 

 (Mundschilde, scuta buccalia) of authors. They have different forms in different genera ; in 

 general, they are single, as in Ophiolepis (PI. XIII, fig. 3), and sometimes they consist of 

 an inner larger and an outer smaller portion, as in Ophiarachna (PI. XIII, fig. 5). In one 

 of these five buccal plates is found, when present, the umbo, a smaU depression in the 

 middle of the plate, the homologue, perhaps, of the madreporiform body of Asteroidea. 



Each interbrachial space terminates in a triangular-shaped body, and the five form 

 a star-shaped opening, having the oral aperture for its centre and the buccal fissures for 

 its rays. The terminal process is a narrow cone which rises high up within the mouth, 

 and forms a jaw (maxilla), armed with numerous calcareous pieces piled upon each other, 

 which perform the part of teeth. Plate XIII, fig. 1, shows the form of the maxillae in 

 Ophioderma longicauda ; fig. 3, in Ophiolepis ciliata ; fig. 4, in Ophiocoma dentata ; fig. 5, 



