III. Lebetodiscus Dickson i. 



1908 545 



The grooves reach the periphery, but do not curve round it or pass 

 over it. They would therefore have been invisible from the under 

 surface. The left posterior groove attained a length of 19 mm. from 

 the actinal pole ; the left anterior and right posterior were about 

 1*5 mm. shorter ; the others are too imperfect to measure accurately. 



, distal ^ 



covering flooring interradial 



Sup.vective Skeleton of Lebetodiscus BieJisoui. 



Fig. 1. — Portion of a ray, showing- relations of covering-plates to food-groove and 

 flooring-plates ; slightly diagrammatised, x C diameters. 



Fig. 2.— Portion of a ray, seen in three-quarter perspective, showing covering- 

 plates and the excavation of the flooring-plates ; slightly diagrammatic, 

 x 20 diam. 



Fig. 3. — Portion of the anterior ray, showing Iioav pores are formed by lateral 

 excavation of flooring-plates ; sketched under the microscope, x 6 diam. 



Fig. 4. — Similar appearances in another part of the same ray, with some covering - 

 plates ; sketched under the microscope, x 6 diam. 



(All drawings made from Bigsby's specimen.) 



The skeleton of the subvective grooves is stout and raised above the 

 general surface to a height of about 1*2 mm. in their proximal regions. 

 The width near their proximal ends is 2*8 mm. ; they taper gradually 

 distalwards, and are rounded off rather bluntly. Each is clearly seen 



