LEPIDASTER GRAYI. 119 



gradually until, when we reach the adambulacralia situate a short distance away 

 from the base of the arm, the whole surface of the ossicle is uniformly swollen. 

 Owing to the displacement of the sixth to the eleventh adambulacralia this is not 

 well seen in the figure. The most distal (the eleventh) adambulacral alone shows 

 an evenly swollen surface. Further details as to the structure of these proximal 

 adambulacralia may be gathered from the study of other portions of the specimen. 

 The armature of spines is preserved on certain ossicles and figured (Text-fig. 72). 

 It is seen that the nose end of the adambulacral carried spines which stretched 

 across the groove. Stout spines were also carried on the transverse ridge. 



The displaced adambulacralia mentioned above show the position and form 

 of the inter- adambulacral muscles. The proximal articulation is situate upon a 

 slightly projecting ridge placed at the inner edge of the ossicle. In consequence 

 of the upward tilt of the articulating surface one can look well down into the 

 muscle-excavation. The ridge in its natural position fits a backward projection 



Text- fig. 72.— Outline drawing of two proximal adambulacralia of Lepidaster grayi, to show the spines 



on the ridge, x 8. 



from the preceding adambulacral, and under this projection is the excavation for 

 the proximal extremity of the same muscle. The small deep hollows for the inter- 

 adambulacral muscles are confined to the inner ends of the ossicles throughout the 

 greater part of the arm. They present a very characteristic appearance. At 

 the distal extremity they disappear, and then there is a weak concavity covering 

 the whole of the articulating surface. 



A side view of a displaced proximal adambulacral is figured (PL VII, fig. 4). 

 The outwardly projecting knob articulated with a ventro-laterale. The proximal 

 ambulacralia are figured (PI. VII, fig. 2). They are flat plates with forwardly 

 projecting pegs which fit into sockets on the preceding plates. A ridge for the 

 separation of the tube-feet is present, but it is very thin and low, suggesting that 

 the tube-feet are much reduced in functional importance. The distal ambulacralia 

 are exposed in arm iv. These are oblong evenly swollen ossicles, and so far as I 

 can see show no trace of a transverse ridge. Their inner end is somewhat 

 swollen, but there is no trace of a concavity for the insertion of ventral cross- 

 muscles. Obviously the articulation with the opposing ambulacralia was very 

 loose, which may account for the easy displacement of these ossicles (see below). 



