428 PROCEEDINGS OF THE ACADEMY OF [May, 



along the ground; and it is jerked forward when the foot is raised, 

 sometimes supported upon the operculum, but quite as often not. 

 The muscles controlling the movement of the foot and of the pro- 

 boscis can evidently act quite independently of each other. 



The external soft parts of T. hilabiata are very pale cartridge buff. 

 There is an ill-defined, flesh-tinted spot on the prol)Oscis, caused by 

 some colored body, perhaps the radula and its sack, shining through. 

 The proboscis has faint annular wrinkles. 



Compared with other land operculate snails we have seen alive, 

 Truncatella is remarkable for the small size of the foot and the 

 extraordinary development of the proboscis. 



The terrestrial prosobranch gastropods show a good deal of diver- 

 sity in dealing with the problem of progression on a dry surface. 

 The Cyclophoridce glide, like aquatic taenioglossate forms. The 

 Ericiidce move by the forward translation of vertical folds, alternating 

 on the two sides of the foot, while the Trimcatellidce step. The 

 gait of Pomatiopsis lapidaria is intermediate between the step and 

 the glide, and serves to show how the gait of Truncatella was probably 

 evolved. The proboscis and oral disk are used exactly as in Trunca- 

 tella, but the foot moves by gliding or sliding, first the fore part 

 moving forward to the proboscis, then the back part. 



Explanation of Plate XIV. 



Fig. 1. — Truncatella hilabiata. Proboscis and foot both in contact with ground, 

 the front part of foot being raised preparatory to forward movement. 



Fig. 2. — Near the end of the forward step of the foot, the shell trailing far behind. 



Fig. 3. — End of the forward step of proboscis, the shell trailing far in the rear. 

 This position is slightly anterior to that shown in fig. 1. 



Figs. 4, 5. — Segmentina obstructa geoscopus, n. subsp. ' 



Fig. 6. — Truncatella bilabiata. End of forward step of the foot, the shell pulled 

 forward. This position is slightly later than that shown in fig. 2. 



