24 THE SHELL. 



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 whilst 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 

 Planorbis (i, 18)], or irregular in their growth, owing to an 

 unhealthy condition. The discoidal Ammonites sometimes show 

 a slight tendencj^ to become spiral, and more rarely become 

 unsj'mmetrical, and have the keel on one side instead of in the 

 middle. Helices occasionally occur with tilted whorls, or even 

 more or less unwound and scalariform (i, 16, 17). 



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 byssus ; and balani sometimes produce strange protu- 

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

 themselves when young. 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 liortensis which got entangled in 

 a nut-shell when j-oung, and growing too large to escape, had to 

 endure the incubus to the end of its days. Fischer speaks of a 

 Helix aspersa, which has entangled in its aperture a younger 

 shell of the same species, which it has soldered fast, thus bearing- 

 two shells. Cailliaud has produced monsters by bringing dead 

 shells in contact with growing ones. 



In the miocene tertiaries of Asia Minor, Professor Forbes 

 discovered whole races of Neritina, Paludina, and Melanopsis, 

 with whorls ribbed or keeled, as if through the unhealthy 

 influence of brackish water. The fossil periwinkles of the 

 Norwich Crag are similarlj^ distorted, probably b}^ 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, and in some localities tend to 

 become hereditary. Bulimus citrinus is as often sinistral as 

 dextral ; and a reversed variety of Fusiis antiquus was more 

 common than the normal form in the pliocene sea. Other shells 

 are normally sinistral, as in many species of Pupa, and the entire 

 genera Clausilia, Ph3^sa, and Triforis. Bivalves less distinctly 

 exhibit variations of this kind ; but the attached valve of Chama 



