86 TUIC NAUTILUS. 



possess {liis type of mnvcmeiit. The rm-tliml of progres ion of Ad- 

 amsielld irrorata Gloyne, CoLjhost ijlna broniiu (C B. A<1.) and 

 Tudora ariiiala (C. 1>. Ad.) lias breii cartd'ully btndied (Voin S|)eci- 

 mens obtained in Jauiaicn during the past summer (^1 910), and an 

 account of tiieir movements will apply in general to all ot the above 

 species.^ 



Unlike' most snails, the entire under surface of the foot is not 

 applied to the surface upon which they move, but they wdk on the 

 edges of the foot only. The foot, when retracted into the shell, folds 

 down the median line of the under surface; and, when protruded for 

 walking, this same form is retained, so that ordinarily only the two 

 outer surfaces are involv(Hl. A wave of contiaction, lifting a portion 

 ot this edge of the l'o(U, proceeds iVom tiie posterior to the anterior, 

 the wave moving foiward altei-nately on the two margins of the foot, 

 and with its advance, first on the right side and then on the left, 

 producing a swaying gait that is characteristic of the movement of 

 all the land operculates above enumerated. The wave m.ovement is 

 quite independent in tlie two sides of the foot, and this is easily seen 

 when the animal is examined from the under side, when crawling 

 orer a surface of glass. As the wave passes off at the anterior end 

 of the foot on one side the succeeding wave begins at the posterior 

 end on the other side, and thus the animal acquires the waddling or 

 swaying gait characteristic of this type of movement. As the wave 

 passes off anteriorly a part of the foot is lifted and moved forward; 

 as the wave appears at the posterior end' a part of the foot is lifted 

 and moved forward also, so that seen from the front or rear espe- 

 cially, the animal seems to be walking upon stumpy legs, and the 

 movement recalls that of an elephant as seen from the rear. The 

 trail made, when moving over dry surfaces (as the animals normally 

 have to when in their native habitat), is double, only the edges of 

 the foot being involved, and this double trail may always be seen 

 when these snails are actively moving about. The details of the 

 movement in the three species that were especially examined is given 

 below. 



Colobostyhis hromiii (C. B. Ad.), Figure 3. This species is larger 



' In a recent paper on the raollusca of Mandeville, Jamaica, by Pilsbry and 

 Brown this mode of progression is briefly described ii.s it was observed ia 

 Colobosty las jay anus rMfilabris (C. B. Ad.) and in Tudora armata (C. B. Ad.). 

 See Proc. Acad. Nat. Sci., Sept., 1910, p. 522. 



