URASTERELLID.E. 



133 



character, of alternate or opposite arrangement of floor-plates, in our definitions of 

 genera or larger groups of pala?ozoic sea-stars. In Blastoidocrinus, the blastids, and 

 all forms in which the growing arm-tip rests against the bibrachials, radials, or 

 terminal plates, the flooring-plates are developed alternately. When one has 

 become well grown and stiffened with stereom it takes up the thrusts against the 

 groAving arm-tip and leaves a space on the opposite side, practically free from 

 compression, where the embryonic new plate and its concomitant structures may 

 assume their proper positions. The subsequent arrangement of these plates is due 

 to other factors, and they may be found alternately placed in one arm whilst 

 oppositely placed in another of the same individual." 



As the arm is followed distalwards the ambulacralia become much squarer in 

 outline. 



The adambulacralia are stout and high. Each has a small nose where it meets 



85 



8-1 



R-- 



y///l^$^ 



z'D. 



R- 



- - - D. 



Text-fig. 84. — Drawing illustrating the change in the slope of the ambulacralia of Urasterella thraivensis 



as they proceed from the mouth-region (P.) distalwards (D.). 



Text-fig. 85. — Diagram illustrating the muscle-insertions on the proximal adambulacralia of Urasterella 



thraivensis. Ap., apical surface ; Or., oral surface ; D., distal surface ; P., proximal surface. 



an ambulacral. The oral faces have well-marked transverse ridges to which were 

 attached long spines (PI. VIII, fig. 3). The possession of the ridge and spines 

 gives some resemblance to the side-shields of the Ophiuroidea. The resemblance, 

 however, is superficial. The Ophiuroid side-shields are but loosely connected, the 

 interadambulacral muscles being only slightly developed or absent. The Urasterellid 

 adambulacralia have strong interadambulacral muscles, which appear to have 

 played a similar part in wriggling movements to the ambulacral (vertebral) 

 muscles of the Ophiuroidea. This is particularly well seen in the case of the 

 adambulacralia near the base of the arms, which have distinct differentiation in 

 their proximal and distal muscle-excavations leading to increased power of the 

 muscle. The proximal excavation is on a forward projecting ridge situate just 

 under the oral face, while the distal excavation is on a backwardly directed 

 projection just above the nose. In consequence of the slope of the ossicles already 

 referred to, the interadambulacral muscles must have run very much as in the 

 diagram given (Text-fig. 85). The contraction of the muscle would tip the ossicles 

 still more towards the mouth and so cause dorso-ventral flexions. Side flexions 



18 



