THE STELLEROIDEA 



275 



The diagnosis of this order is necessarily vague, as it includes 

 a series of simple forms, among which there are great divergences in 

 structure. One living genus included in this order has no ventral 

 arm-plates, but a small ambulacral furrow, and thus agrees with the 

 Palaeozoic genera, for which Stiirtz in 1885 proposed the family Ophio- 

 encrinasteriae. The main character of the order is the simple articulation 

 of the arm ossicles. The ambulacral plates are always opposite instead of 

 being alternate, as in the Lysophiurae. In some of the most primitive 

 genera the two ambulacral ossicles in an arm segment are separate, as in 

 Ophiurina, or incompletely joined, as in Ophiohelus. In the Lapworth- 

 uridae they are firmly united, and in the recent genera Ophiomyces and 

 its allies they are specialised into definite ambulacral ossicles, with a 

 simple ball-and-socket articulation. 



FAMILY 1 . OPHIURINIDAE. Streptophiurae without ventral arm-plates 

 and with separate ambulacral ossicles. Genera Ophiurina, Stiirtz, 

 Devonian. Tremataster, Worthen & Miller. 



FAMILY 2. LAPWORTHURIDAE. Strepto- 

 phiurae without ventral arm-plates or buccal 

 shields. Ambulacral ossicles fused, but their 

 articulating surfaces are plain. Genera Lap- 

 wortkura, Greg., Silurian, W. of England ; Fur- 

 caster, Stiirtz ; Palastropecten, Stiirtz. The oral 

 armament is typically Ophiuroid. The struc- 

 ture in the genus Furcaster is shown in Fig. 

 XXXI. ; mouth-frames, small jaws, jaw plate, 

 and teeth are all present. 



FAMILY 3. EOLUIDIDAE. Streptophiurae 

 with ambulacral ossicles united to form verte- 

 bral ossicles. Ventral arm -plates present, 

 but there are neither buccal shields nor 

 dorsal arm-plates. Genera Eoluidia, Stiirtz ; 

 Eospondylus, Greg. ; Miospondylus, Greg. ; 

 Ayanaster, Miller & Gurley (syn. Ophiopeye, 

 Bohm) ; Cholaster, Worthen & Miller. 



FAMILY 4. ONYCHASTERIDAE. Strepto- 

 phiurae with well-developed vertebral ossicles, 

 and very flexible, unbranched arms. There 

 are no external arm -plates, the integument only containing granules. 

 Genus Onychaster, Carboniferous, Illinois, has previously been included 

 among the Euryalid group, but its ambulacral ossicles (as shown by Meek 

 and Worthen's later figures, Geol. Surv, Illinois, vol. v., 1873, pi. xvi. 

 fig. 3d) are Streptospondyline. 



FAMILY 5. EUCLADIIDAE. Streptophiurae with contorted branching 

 arms. Five pairs of large plates round the centre of the side exposed in 

 the fossil have been regarded either as jaws or as radial shields. Madre- 

 porite on same side as the plates. Arms have no external arm-plates but 

 a granular integument. The type-species, Eucladia Johnsoni, was care- 

 fully figured and described by H. Woodward in 1869. Owing to its 

 flexible, branched arms, and soft integument, it has generally been 



FIG. XXXI. 



Furcaster palaeozoicus, Stiirtz, 

 showing skeletal elements of the 

 arms and oral armament, ad, 

 adambulacral ossicles bearing 

 spines ; am, ambulacral ossicles ; 

 j, jaw ; j.p, jaw plate ; m.f, mouth- 

 frame ; t, teeth. (After Stiirtz.) 



