STUTTCTTJRE AND PHYSIOLOGY OF THE MOLLUSCA. 37 



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

 shell (as in the vertebrate skeleton) indications are afforded of 

 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 whorls 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 specimens of planorbis found at Eochdale), or irregular 

 in their growth, owing to an unhealthy condition. The discoidal 

 ammonites sometimes show a slight tendency to become spiral, 

 and more rarely become unsjTnmetrical, 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 their 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 hyssus ; and halani sometimes produce strange protu- 

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

 themselves when young.* 



In the miocene tertiaries of Asia Minor, Professor iPorbes 

 discovered whole races of neritina, paludina, and melanopsis, 

 with whorls ribbed or keeled, as if through the unhealthy in- 

 fluence of brackish water. The fossil periwinkles of the 

 Norwich Crag are similarly distorted, probably by the access of 

 fresh water ; parallel cases occur at the present day in the 

 Baltic. 



Reversed shells. Left-handed or reversed varieties of spiral 

 shells have been met with in some of the very common species, 

 like the whelk and garden- snail. Bidimus citrinus is as often 

 sinistral as dextral ; and a reversed variety of fusus antiquus 

 was more common than the normal form in the pliocene sea. 

 Other shells are constantly reversed, as pyrula perversa, many 

 species of pupa, and the entire genera, clausilia, physa, and 

 triforis. Bivalves less distinctly exhibit variations of this 



* In the British Museum there is a helix if errexf r«s (Chemn.) with a small stick 

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

 in his coilection, a helix hortensis which got entangled in a nut-shell when young, and 

 growing too large to escape, had to endure the incubus to the end of its days. 



