18 



BRITISH PALEOZOIC ASTEROZOA. 



(a) The deep excavations for the insertion of the writhing muscles. Ventrally 

 these excavations are more prominent proximally than distally (Text-fig. 14). 



(/>) The narrow ridge left by the deep excavations (Text-fig. 15 and PI. I, 

 fig. 3). The adambulacralia (side shields) articulate with this ridge. They are not 

 bound to the anibulacralia (vertebrae) l)y muscles as in the Asteroidea. 



(c) The cup for a tube-foot is not formed by two succeeding ossicles as in 

 the Asteroidea, but is entirely situated on one vertebra. 



{d) The manner in which the narrow branch canal from the radial water- 

 vascular canal perforates the ossicle and runs a tortuous course (Text-fig. 15) 

 before emerging into the cup (Text-fig. 14). 



(e) The articulations between the vertebrjB (Text-figs. 10 and 17). These 

 are very complicated in the majority of Recent Ophiuroidea. They are described 

 by MacBride (43, p. 481) as follows : " On the proximal surface of the central 

 portion of the vertebra is a central knob and two ventro-lateral knobs, a median 



16 17 



/li^ 





/ ^ 



\ V 



^ JMi 



hV 



X^ 



L^ 



Text-figs. 36 and 17. — Proximal and distal views of single vertebrae of Ophiarachna incrassata (after Ludwig). 



ventral pit and two dorso-lateral pits, and on the distal surface there are pits corre- 

 sponding to the knobs on the proximal side and vice-versd." 



"We shall see that the Palseozoic Asteroidea allow us to trace the gradual 

 evolution of these complicated structures. 



The Structure of the Arm in the Palaeozoic Asterozoa. 

 The Views of Previous Observers. 



(a) On the Form of the Primitive Amhulacralia. — Jaekel alone (34) of previous 

 observers has drawn attention to the correct shape of the ambulacralia in the early 

 Palaeozoic Asterozoa. His figure of Siluraster shows the characteristic oblong form 

 which these plates assume. A reference to Text-fig. 18 (p. 20) of this monograph 

 shows similar plates in a British Ordovician species, TJranaster elkas, n. sp. The 

 plates form a complete floor to the ambulacral groove. There are no pores for the 

 passage of ampullge. The H-shaped ridges already refered to (p. 14) run across 

 the top[of each ambulacral, and divide the groove into a series of deep depressions 

 in which were placed the tube-feet. Many investigators, owing to the poor state 



