134 BRTTTSH PALvEOZOTC ASTEROZOA. 



would be produced by the action of the muscles of one side only. The position of 

 the arms of Oalliasterella, figured Text-fig. 109, p. 167, shows that the arms of 

 relations of Urasterella had considerable power of rolling up, and one cannot doubt 

 this for Urasterella itself. Somewhat similar interad ambulacra! muscles may be 

 found in recent Asteroidea. It is the large size of the adambulacralia in the 

 Urasterellidpe which gives the muscles a special significance. 



The two muscle-insertions of the adambulacralia appear to be more evenly 

 hollowed from the middle of the arm to the distal extremity. 



The inner adambulacral face is flat in the proximal region of the arm. Distal- 

 wards it is distinctly ridged. The ridges, as usual, helped to form the walls of the 

 depressions for the tube-feet. 



If the groove be open the inner face appears to form a knee-bend where it 



86 



87 



0. 



A 



m. 



Text-fig. 86. — Wash drawing of an interradial angle of Urasterella thraivensis. Ad., adambulacralia 



I.M., infero-marginalia ; M. P., mouth-angle plates ; O., odontophor. x 4. 

 Text-fig. 87. — Wash drawing of an interradial angle of Urasterella thraivensis viewed from inside the 



mouth (adoral wards). Ad., adambulacralia; Am., first ambulacral ; M.P., mouth angle plates; O., 



odontophor. x 4. 



joins the oral face (PI. XI, fig. 8). Usually the groove is not found open, but 

 almost if not completely closed. It is so figured in the reconstructions given 

 (Text-figs. 89, 91, 95). 



The crests of the adambulacral ridges are occupied by raised perforated 

 pustules, to which doubtless stout spines were attached. No spines have been 

 observed in this species, but they ma}' be seen in U. ruthveni and U. ruthveni, var. 

 leintwardinensis. 



Hudson suggests (op. cit., p. 134) that the adambulacral spines of I . medusa 

 are " minute paxillae, each pedical bearing two spinelets." 



Structure <>/ tii<> Moutk-parls (Text-figs. 80, 87). — The mouth-parts are strikingly 

 similar in both the English and the American species of the [Jrasterellidae. Text- 

 fig. 8(3 shows a view of the interradial angle of U. thraivensis. The mouth-angle 

 plates are prominent and triangular flat ossicles in series with the crested adambula- 

 cralia. Immediately distal to and within the angle of the arm is the odontophor 

 (the axillary interbraehial of Schuchert and Hudson). Behind this are outlined 



