98 Dr. J. F. Gemmill. [Jan. 17, 



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Fig. 6. — Photograph (natural size) of track left by a small urchin heavily loaded and 

 travelling under water on a horizontal surface covered -with a thia layer of 

 plasticene. Numerous very short steps were taken. The slanting of the dents 

 indicates the direction of movement, viz., towards the foot of the page. 



V 



Fig. 7. — Photograph (natural size) of track left by a larger urchin completely immersed 

 under water and induced to travel up an inclined plane of smooth plasticene 

 in the manner described. The marks of the teeth are slanting but not deep. 

 The direction of movement was towards the foot of the page. No mark 

 at all has been left by the anterior tooth, i.e., the tooth on the side towards 

 which the urchin is travelling. 



