22 THE SHELL. 



the time sinking into a torpid condition ; but these animals have 

 the power, at the first recurrence of damp weather, to remove 

 this cover, which is not the case with the hard shelly secretions 

 which cover up the mouth of the shell of Rhizochilus."* 



Notwithstanding the decided opinion given by Dr. Gray that 

 the self-immurement of the Rhizochilus is permanent, I cannot 

 help thinking that it only continues during a period of hiberna- 

 tion, and as many mollusks have the power of absorbing away 

 partitions in their shells, as well as parts of the columella and 

 the interior thickening of the outer lip, it appears to me that his 

 argument that the hardness and thickness of the prison-walls 

 would prevent escape, can scarcely be sustained. 



In certain species of Calyptrsea and in Hipponix the foot is 

 atrophied. The mantle secretes not only the shell which covers 

 the animal, but also, from its ventral face a calcareous base 

 attached to some foreign body, and to this the mollusk adheres 

 by a horseshoe-shaped muscle. 



Magilus (xlv, 52) resembles the Teredos in constructing a 

 tube, and for the same purpose. In Magilus the tubular growth 

 is, however, a continuation of the growth of the shell margin, 

 but changed in form, and in this respect it is more like the 

 Aspergillum, which builds out a tube from the valves which lay 

 open near the end and form part of its circumference. 



A more or less irregular unrollment of the spiral takes place 

 in Yermetus (Ixvii, 68) and its allies, and to a less degree in 

 certain Cuban Cylindrellse and some other terrestrial mollusks. 

 In the terrestrial genera Hypselostoma, Tomigerus and Anos- 

 toma, the adult character is proclaimed by a very curious change 

 in the progression of the spiral by which the mouth of the shell 

 is turned upwards to its superior or spire face : so in certain 

 South American Bulimi the saccate body-whorl of the adult is 

 deflected from the axis of the spire-whorls. 



Sculptured shells, particularly Ammonites, and species of 

 Rostellaria and Fusus, often become plain in the last part of 

 their growth. But the most characteristic change is the thick- 

 ening and contraction of the aperture in the univalves. The 

 young cowry has a thin, sharp lip (Ixi, 97), which becomes 

 curled inwards, and enormously thickened and toothed in the 

 adult ; the Pteroceras (lix, 63) develops its scorpion-like claws 

 only when full-grown ; and the land snails form a thickened lip, 

 or narrow their aperture with projecting processes, so that it is 

 a marvel how the}' pass in and out, and how they can exclude 

 their eggs. 



Yet at this time they would seem to require more space and 

 accommodation in their houses than before, and there are several 



* Ann. Mag. Nat. Hist., 2d ser., vii, 477, 1851. 



