14 



BRITISH PALAEOZOIC ASTEROZOA. 



aclambulacral spines as tlie " aclainbulacral armature." He admits, however, that 

 they can be subdivided into (a) " groove spines," that is, the row of spines Avhich 

 are always present as a protection to the tube-feet when these are withdrawn into 

 tlie groove, and (b) " snb-ambuhicral " spines, exterior to tlie groove spines. 

 These may be absent or only represented by grannies. I propose to nse these 

 terms in my later descriptions, as the subdivision appears to be of considerable 

 importance. 



If we now examine the ambnlacralia, the most conspicuous features are the 

 cross-ridges which separate the depressions for the tube-feet. Each cross-ridge is 

 surmounted by a second ridge running parallel with the ambulacral chaimel and 

 excavated on its inner face by a concave depression for the insertion of the ventral 

 cross-muscles. The two ridges appear — |-shaped. This is another feature of 

 great importance in the study of fossil forms (compare Text-fig. 18, p. 20). 



PROXIMAL. 



ip. 



Text-fig. 8 (on left). — Side view of two anibulacralia of Stellaster childreni. Abbi-eviations as in Text-fig. 6. 

 Text-fig. 9 (on right). — Side view of an ambulacral and adarabtilacral of Archastcr typicus. Abbi-eviations 



as in Text-fio:. 6. 



(iii) Tlie Ann in Dorsal Aspect. — -The ambnlacralia and adambulacralia can only 

 be seen in apical aspect after the dorsal portion of the arm and disc has been cut 

 away and an opening thus made into the body-cavity. The ambnlacralia viewed 

 from this aspect (Text-fig. 0) appear in full length. The point of junction of 

 opposite components of ambulacral pairs is at the bottom of cups formed by 

 upwardly projecting concave ridges which served as the attachment for the dorsal 

 cross-muscles. The ambulacral pore (for the passage of the ampullge) can be seen 

 between the hollowed-out portions of the ambnlacralia. Exterior to the i)ores are 

 observable the bases of the ambnlacralia, each formed from a thickened main 

 portion, the apopliysu, and a small forwardly directed }{:iug. 



(iv) 7V/(" structure of the Aiiihnlncralia and Adambulacralia as seen after ^laceration 

 and, Displacement of the Ossicles. — Further details may be observed after maceration 

 of the ossicles in potash. Text-fig. 5 shows an isolated ambulacral and adam- 

 bulacral viewed from the oral side. The ambulacral is, however, laid almost along its 

 extreme length instead of " on end " to form the component of an arch. The true 



