426 PROCEEDINGS OF THE ACADEMY OF [May, 



THE METHOD OF PROGRESSION IN TRUNCATELLA. 

 BY HENRY A. PILSBRY AND AMOS P. BROWN. 



Early in August of 1913, when one of us (Brown) was collecting 

 fossils along the shores of Willoughby Bay, Antigua, upon turning 

 over a piece of limestone a few feet above high-water mark, he came 

 across a colony of Truncatella bilabiata Pfr. They were very plentiful 

 under the slab, and those that were not disturbed by the lifting of 

 the stone were seen to be in motion. Others at once retracted the 

 body into the shell along with the operculum and assumed the 

 appearance of dead shells. On observing those that continued to 

 move it was at once noticed that they do not employ the same 

 method as the rapidly moving Colohoshjlus and Tuclora already 

 observed in Jamaica, nor that of the Cistula observed in Antigua, 

 but seem to adopt the method of the "measuring worm" in their 

 progression. A number of the shells with the living animal were 

 collected in a specimen bottle, and upon returning to the town of 

 St. John's they were kept under observation for some time. In 

 fact, upon returning to Philadelphia some weeks later, most of them 

 were still quite lively and moved about actively. It was from 

 these survivors that the figures were drawn. Before leaving St. 

 John's, notes upon the method of motion were made. These obser- 

 vations were repeated in Philadelphia. 



When disturbed or startled, the animal at once withdraws into the 

 shell and closes the opening by the operculum, which in fact is 

 drawn in beyond the lip. They then resemble pieces of stone or 

 fragments of dead leaves, and without close observation tl\ey would 

 escape notice. After they are left to themselves for a short time, 

 the animal protrudes the operculum, with the foot and proboscis, 

 which latter at once begins to feel about until it encounters some 

 firm substance, when the foot is fully protruded. The foot is a 

 squarish pad of about 1 scjuare millimeter in area. The proboscis 

 may be extended to 1^ mm. or more. It is waved to and fro until 

 it encounters the surface over which the animal moves, bending 

 downward at the same time and presenting in front view a certain 

 ludicrous resemblance to the head of a moose, which resemblance is 

 enhanced by the blunt expanded snout of the proboscis and by the 



