EOACTINID^E. 187 



" b " muscles. The excavations for these are at the outer ends of the calcite 

 bars, that is at the points of greatest power for lateral flexion. Between the 

 excavations and the radial line the bars are in contact, and any lateral flexions will 

 naturally cause the surfaces of contact to rub against each other. The greatest 

 rub will be just at the angle on the inner side of the muscle-excavations, for this 

 will be the fulcrum of the lever movements. Here we get a forwardly projecting 

 peg (Ar. in figure). It is at this point that the dorsal ball and socket of the 

 Ophiuroid vertebra develop just as the ventral ball and socket develop in relation 

 to the overlap of the oral surface of this same bar (see p. 182). We find a similar 

 development of the apical surfaces of the ambulacralia in the Platanasteridse. In 



A 



Ar. 



A 



m. 



DIM. 



Text-fig. 127. — Wash drawing of the apical surface of the proximal ambulacralia and neighbouring plates 

 of Schuchertia icenlocH. A., ascending process of the mouth-angle plate; Ad., adambulacral ; Am., 

 ambulacral ; Ar., forwardly projecting peg; D.L.M., position of dorsal longitudinal nruscle ; I.M., infero- 

 marginal. x 12. 



the Ordovician representative the bars are closely touching (Text-fig. 120, 

 p. 175); in the later Devonian species excavations and pegs arise (Text-fig. 122, 

 p. 177). 



Stellaster equestris may be taken as a recent Asteroid to show similar 

 phenomena. Text-fig. 128 gives an appearance which we shall see in many 

 Ophiuroidea. The "b" muscles are well developed. The proximal excavation for 

 these muscles is situate upon a raised tongue, which projects forwards and over- 

 hangs the backwardly sloping distal excavation. The raising of the surface of the 

 calcite into a tongue causes the formation of a median groove running across the 

 ossicle. The slant causes the ossicles to overlap, as Viguier has remarked (100, 

 p. 53), " just as tiles on a roof." The surfaces of contact are shaped much as in 

 Schuchertia, with a forwardly projecting point (Ar.). A profile view (Text-fig. 

 129) shows that at the outer side of the surfaces of contact there are wearing 

 surfaces (analogues of the ball and socket of the Ophiuroidea) not hitherto noticed. 



