CNEMIDACTIS GIRVANENSIS. 



1 59 



completely exposed. The flat-topped appearance of the exposed ossicles is very 

 characteristic of the species. Each fits tightly with its opposite and succeeding 

 neighbour, the only gap being at the outer extremities where small triangular 

 openings may very occasionally be observed. It may be that ampullae penetrated 

 the dorsal cavity through these openings. The tight firmly-fitting ambulacralia 

 almost give one the impression of bricks set in mortar. The very occasional 

 glimpses one obtains of adjoining faces show steep sides without muscle-depressions. 

 Cnemidactis is obviously not a wriggling form. 



Apical Surface of Disc (Plate XI, fig. 1 ; Plate XII, figs. 1, 2). — The disc is 

 covered by a number of small spicular plates which do not appear to have any 

 regular arrangement. I have been unable to discover any trace of a madreporite- 



Text-fig. 102. — Plan of ossicles, seen in oral view, of an interradial angle of Cnemidactis girvanensis. 

 Ad v> first adambulacral ; Ad,., second adambulacral ; M.P., month-angle plate ; O., odontophor ; 

 T., torus, x 10. 



Mouth-parts. — Several of the specimens give perfect views of the mouth-parts 

 in their natural position. Others show the same parts slightly dislocated, so that 

 views can be obtained of individual ossicles. It is thus possible to describe this 

 region in considerable detail. 



Oral Aspect of Mouth-parts (Plate XI, fig. 2 ; Text-fig. 102).— The odontophor 

 is a very conspicuous ossicle with a characteristic shape. The proximal end is 

 wedge-shaped, the wedge fitting in between the angle made by the distal extremity 

 of the mouth-angle plates. Distal to the wedge is a slight convexity upon which is 

 fitted the first adambulacral. Beyond this again is a wide concavity in which sits 

 the second adambulacral. A distinct peculiarity of the adambulacralia is the 

 position of the nose, which is not proximal to a median transverse line as is usual 

 amongst the Asterozoa (compare Text-fig. 18, p. 20) but distal. It will also be 



