STRUCTURE AND PHYSIOLOGY OP THE MOLLUSCA. 45 



the shell truncated, or deeoUated. This happens constantly with 

 the truncatellcB, cyllnclrellce, and bulimus decollatus ; amongst the 

 fresh-water shells it depends upon local circumstances, but is very 

 common \N\i]i 2nrena and ceritJddea. 



Forms of shells. These will be described particularly under 

 each class ; enough has been said to show that in the moUuscan 

 shell (as in the vertebrate skeleton) indications are alforded o 

 many of the leading affinities and structural peculiarities of the 

 animal. It may sometimes be difficult to determine the genus of 

 a shell, especially when its form is very simple ; but this results 

 more from the imperfection of our technicalities and systems, 

 than from any want of co-ordination in the animal and its shell. 



Monstrosities. The whiiis of spii-al shells are sometimes 

 separated by the interference of foreign substances, which adhere 

 to them when young ; the garden-snail has been found in this 

 condition, and less complete instances are common amongst sea- 

 shells. Discoidal shells occasionally become spiral (as in speci- 

 mens of planorhis found at Eochdale), or ii'regular in their 

 gi'owth, owing to an-unhealthy condition. The discoidal ammo- 

 nites sometimes show a slight tendency to become spiral, and 

 more rarely become unsymmetrical, and have the keel on one 

 side, instead of in the middle. 



All attached shells are liable to interference in their growth, 

 and malformations consequent on theii' situation in cavities, or 

 from coming in contact with rocks. The dreissena polymorpha 

 distorts the other fresh-water mussels by fastening their valves 

 with its bi/ssiis ; and halani sometimes produce strange protube- 

 rances on the back of the cowry, to which they have attached 

 themselves when young.* 



In the miocene tertiaries of Asia Minor, Professor Forbes 



* In the British ^luseuin there is a helix terrestris (chemu.) with a small 

 stick passing through it, and projecting from the apex and un)bilicus. Mr. 

 Pickering has, in his collection, a helix hortensls which got entangled in a nut- 

 shell when young, and growing too lai'ge to escape, had to endure the incubus 

 to the end of its days. 



