STETJCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 45 



the shell truncated, or decollated. This happens constantly with 

 the truncatella, cylindrella, and biiUmus decoUatus ; amongst the 

 fresh-water shells it depends upon local cii'cumstances, but is very 

 common wiih.pirena and cerithidea. 



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 afforded 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 forai 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 whii'ls of spiral 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 planorbis found at Eochdale), or irregular 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 byssus ; and balajii 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 Porbes 



* In the British Museum there is a helix terrestris (chemn.) with a small 

 stick passing through it, and projecting from the apex and umbilicus. Mr. 

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

 shell when young, and growing too large to escape, had to endm-e the incubus 

 to the end of its days. 



