V. Steganoblastus. 1914 199 



depressions, cross this ridge at right angles, and so break it up into 

 a row of rounded eminences, producing a toothed appearance. 



At the distal end of the groove the floor-plates rapidly decrease in 

 size, and this series is rounded off before the actual lip of the groove 

 is reached. The distance between the end of the floor-plates and the 

 lip seems to vary in the different radii. That in which best results 

 have been obtained by preparation is the anterior groove of specimen A 

 (PI. XV, Pig. 3), but this is confirmed by other grooves so far as it 

 has been possible to clean them. That the distal end of the groove 

 was quite free of floor-plates is proved by the continuation of a double 

 row of pits over the lip and along the middle of the groove, up to the 

 end of the floor-plates. The sutures bounding these terminal floor- 

 plates are distinct all round, and the floor-plates are seen resting 

 on the grooved surface of the radial. Apparently, however, the radial 

 does not actually pass right under the floor-plates, but is cut away 

 beneath their adradial region, so that the pores between the floor- 

 plates pass through into the thecal cavity. 



The floor-plates in this part of the groove at any rate appear to 

 have their outer boundary at the level of the pores. Owing to the 

 close union of the floor-plates, not only inter se, but also with the 

 radials and adambulacrals, it is impossible to trace any bounding 

 suture along the edges of the groove. It seems, however, a legitimate 

 inference that a generally similar relation of the floor-plates to the 

 adjacent thecal elements prevails, or did in earlier stages prevail, all 

 the way T up the groove ; at all events, it is clear that the floor-plates 

 and adambulacrals are independent morphological elements, and this 

 is a point of much theoretical importance. 



At the distal end of the groove it is plain that the floor-plates of 

 one side alternate with those of the other side. 



The Cover-plates are best preserved in specimen B (PL XV, 

 Tigs. 2, 5 ; Text-fig. 5). In no case are they retained as far as the 

 distal end of the groove, but there is no reason to doubt that they 

 were continuous from the tegmen to the extreme end of the floor- 

 plates. In the upper half of the right anterior groove, starting about 

 one plate below the distal end of the oro-tegminal, there are nine 

 cover-plates in a length of 5 mm. The proximal of these plates has 

 a length of 25 mm. ; the distal one a length of L8 ram. Nearer 

 the oral pole the cover-plates are more irregular in shape and size, 

 especially in this r. ant. radius of this individual. Over the greater 

 part of the groove the cover-plates alternate regularly and meet in 

 a zigzag suture ; but at the proximal end smaller plates may be 

 intercalated along the middle line. In specimen A there are no 

 such intercalated plates, but the median suture in the proximal region 

 becomes curved and interlocking (Text-fig. 6). 



The relation of the cover-plates to the floor-plates appears to be 

 quite regular. There is a cover-plate to each floor-plate, and, so far 

 as can be ascertained after prolonged preparation and study, the 

 sutures between the cover-plates coincide with those between the 

 floor-plates. Thus the pores, which, as already stated, lie just below 

 the attachment of the cover-plates, open under the sutures as in 

 JEdr toaster. 



