THE NAUTILUS. 87 



than either of the other two examined, the shell measuring 16 mm. in 

 height, with a greatest width of 12 mm. The foot is ashy-gray, the 

 rostrum darker, the eyes black, and the tentacles a bright aurora-red. 

 Its movements, while rapid as compared to ordinary snails, are more 

 deliberate than those of T. armata or A. irrorata. The length of the 

 foot when crawling is generally 9 or 10 mm.; the width varies from 

 5 to 4 mm., being narrower when the movement is most rapid. The 

 waves proceed from the posterior to the anterior alternately on either 

 side, requiring about 7 to 8 seconds for the passage of the wave the 

 entire length of the foot, or the wave moves forward about 1.5 mm. 

 per second. The alternate waves on the two lobes of the foot follow 

 each other with an interval of about 5 seconds, or they are 6-7 mm. 

 apart on the two lobes when both waves can be seen together. The 

 rate of progression is quite rapid, varying from 2.5 to 3 mm. per 

 wave, or 12 double waves were observed to advance the animal 70 

 mm., which is an average rate of progression per minute. The sur- 

 face of contact of the edge of the foot involved in the movement 

 varies from 1 mm. wide on each side with a median interspace of 2 

 mm., when moving on dry or dusty surfaces, to 1.5 mm. on each 

 side with a somewhat smaller interspace, when moving on a smooth, 

 non-absorbent surface like glass. The trail is notably double there- 

 fore, and the two parallel marks are somewhat irregular in outline. 



When moving, the shell is carried balanced on the operculum, and 

 to obtain this balance requires some care on the part ol the animal, 

 but once the shell is adjusted to the proper position the animal moves 

 evenly, the shell swaying from side to side from the passage of the 

 alternate waves of contraction of the foot. The balancing of the 

 shell on the operculum was common to all of the three species exam- 

 ined, and undoubtedly it seems to distribute the weight more evenly 

 on the 1'oot, at the same time raising the shell quite clear of the sur- 

 face on which the animal is moving. 



As the foot of this species is large, the character of the wave 

 movement may be readily studied. A part of the edge of the foot is 

 raised from the surface on which the animal is moving, involving in 

 the fold produced about 3 mm., with a clear space of 1 mm. wide, 

 and the fold is usually 2.5 mm. high witli a clear opening of more 

 than 1 mm. This hiatus moves forward at the rate of 1.5 mm. per 

 second, and when it reaches the anterior end of the foot this edge is 

 thrust forward as it is raised, so that when it is applied again to the 



