STRUCTURE AND PHYSIOLOGY OF THM MOLLUSCA. 37 
each class ; enough has been said to show that in the molluscan 
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 Rochdale), 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 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 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 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 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, 
lke the whelk and garden-snail. Bulimus 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. Biyalyes less distinctly exhibit variations of this 
* In the British Museum there is a heliz terrestris (Chemn.) with a small stick 
passing through it, and projecting from the apex and umbilicus. Mr. Pickering has, 
in his collection, a helix hortensis which got entangled in a nut-shell when young, and 
growing too large to escape, had to endure the inenbus to the end of its days, 
