234 AN INTRODUCTION TO THE STUDY OF FOSSILS 



effectual in protecting such timbers. The various species of 

 Teredo are nearly always found within some hard vegetable 

 substance. 



1. Sketch exterior of one valve. 



2. What is the food of Teredo ? How is this procured ? 



,3. Does it digest the wood eroded in the formation of its 

 burrow ? 



CLASS C, GASTROPODA 



Type of class, Biisycon canaliculatus (Linne) (Figs. 102-104). 



This species is one of the largest gastropods on the Atlantic 

 coast, often attaining a length of six inches. It is especially 

 abundant on sandy bottoms, at or below low tide level, where 

 it may be found slowly plowing its way through the sand or 

 lying with the foot partly buried. It ranges from Cape Cod 

 southward, favoring especially the New Jersey coast and Long 

 Island Sound. It has Uved along the Atlantic coast since Mio- 

 cene times. 



The soft body of the animal is inclosed within a single cal- 

 careous shell, somewhat pear-shaped and consisting of six coils 

 or whorls. At one end the shell is prolonged into a slender, half 

 cylindrical canal. As the animal moves the canal projects for- 

 ward and the apex of the whorls backward. Thus it is seen that 

 the canal is at the anterior end of the shell and the apex of the 

 whorls at the posterior. 



Viewed in the position assumed when feeding, there is visible 

 externally a head with two tentacles or feelers and a large foot, 

 which forms the whole ventral aspect of the animal. When the 

 shell is removed, the large elongated body mass, the visceral 

 spiral, is found to extend backward from the upper side of the 

 foot. This, like the shell in which it was lodged, is twisted into 

 a coil. A delicate colorless mantle surrounds the visceral spiral, 

 fitting closely both anteriorly and posteriorly but arching away 

 in the middle, leaving a wide mantle (pallial) cavity. Margi- 

 nally, the mantle is thickened and at one place is produced into 



