160 



BRITISH PALAEOZOIC ASTEROZOA. 



noticed that both the mouth-angle plates and the first adambulacralia are very 

 small. 



A breast-plate-shaped torus, very large in surface view, projects well into the 

 mouth-cavity. It is surrounded by spines. An examination of the photograph 

 given PI. XI, fig. 2, shows that the five tori and their spines completely close the 

 mouth -cavity. 



Adoral Aspect of the Mouth-parts (Plate VIII, fig. 6 ; Text-fig. 103).— I have 

 made a careful model of a portion of the mouth-region. This has been photo- 

 graphed in such a position that one is looking at it from inside the mouth-cavity. 

 A key-drawing to the photograph is given as the Text-figure. The first feature 

 one notices is that the proximal ambulacralia are very large as compared with the 

 mouth-angle plates, that is, the mouth-parts are of the "ambulacral type" of 



R 





0. 



Text-fig. 103.- — Outline drawing as key to the photograph of the model, PI. VIII, fig. 6, showing an 

 adoral view of the plates in the mouth-region of Cnemidactis girvanensis. Ad v , first adambulacral ; Ad. 2 

 second adambulacral ; M.P., mouth-angle plates j O., odontophor ; T, torus, x 6. 



Ludwig. The second point is the sharp dip of the ambulacralia into the mouth- 

 cavity. In consequence, the proximal tube-feet must have been well inside the 

 mouth as in the Ophiuroidea. The large torus is another point of similarity with 

 the Ophiuroidea In this latter class, however, the torus is always firmly attached 

 to the mouth-angle plates throughout its depth. In Cnemidactis the attachment 

 seems to have been slight and only at the distal end of the torus. At any rate the 

 tori are always found in this position, and are so regularly arranged that it is 

 difficult to suppose that they were secondarily pushed there after death. 



The first ambulacral is a large plate as in Urasterella. The ridge separating 

 the first and second tube-feet runs in an oblique direction across the plate. 



Apical View (Plate VIII, fig. 5). — The apical view is especially interesting 

 because of its close resemblance to that of U. medusa (PI. VIII, fig. 4). The only 

 distinction appears to be, that there has been a great fusion of the ambulacralia 

 in Cnemidactis. The first three pairs are usually found so that they form a thick 

 solid bar which stretches from one odontophor to another. A similar fusion in 



