98 New York State Museum 



of the bivalve mollusks are particularly adapted by their 

 compressed and hatchet-shaped forms for burrowing. 

 The Razor Shells (Sol en) and Sword Razors (Ensen) 

 have elongate valves which are oval in cross-section, and 

 these bivalves and similar forms burrow rapidly and 

 deeply. The common Soft-Shell Clam or Gaper (Mya 

 arenaria) is also a deep burrower. Such deep burrowers 

 as these have a lengthened siphon and hence a prolonga- 

 tion of the posterior part of the shell. The valves of the 

 shell are not closed and serve only as fenders against the 

 lateral pressure of the surrounding material, sand or silt. 

 Animals that seek shelter from the strain in quieter 

 waters or deep burrows, develop less heavy and solid 

 shells. Mollusks of the sand, however, that are not deep 

 burrowers are subject to considerable pressure from the 

 shifting of the surrounding loose material and they have 

 in many cases developed heavy, more or less globular 

 shells, as seen in the Cockles (Cardium, Isocardia), the 

 Little Necks or Hard-Shell Clams (Venus mercenaria) 

 etc. Globular forms that are shallow burrowers often 

 have ridges on the shell or other outgrowths that firmly 

 anchor the animal. The Spiny Cockle, which is also 

 known as Red Nose because of its bright red foot, has 

 spines recurved in the direction of its tube which moor 

 it securely. All of the cockles are more or less ribbed 

 and a number also have spines to a certain degree. Venus 

 verrucosa has habits similar to those of the Spiny Cockle 

 and its ridges serve the same purpose as the spines of the 

 latter. The Pelican's Foot (Aporrhais pes-pelicani) of 

 European waters, and its western relative (A. occidenta- 

 lis) are gastropods with heavy shell, living on the surface 

 of the bottom. The shell has a winglike extension which 

 seems to serve as a sort of counterpoise that enables a 



