MOLLUSCOUS ANIMALS. 29 



Before we proceed further it is desirable to inquire what other kinds 

 of characters are available in the present state of our knowledge for the 

 arrangement of Gasteropod Mollusks, and how far we can determine 

 their comparative importance. One of the most obvious characters is 

 derived from the shell itself," its presence or absence, its form, its sub- 

 stance, and its colouring. It is now universally agreed that shells con- 

 sidered without reference to the animals are mere play things altogether 

 destitute of scientific interest, and if valued for their beauty or variety 

 destitute of higher claims on our attention ; but when we consider them 

 as a part of an animal — -a hard deposit on his surface, moulded on hig 

 form, and expressing his external distinctions, we cannot but expect 

 that the study of the hard covering may be also connected with that of 

 the creature— that we may learn to make what can be well preserved 

 an index to much which we have but occasional opportunities of exam- 

 ining, and cannot well retain for reference — nay, even from comparison 

 of the shells, to know the structure of many animals whose organization 

 we have no opportunity of examining at all, though their shells may be 

 in our collection. It is true indeed that form alone is not a constant 

 and certain index to structure, and cases occur in which shells might 

 be placed beside one another, from their very close resemblance, though 

 when we are acquainted with the animal we find that they really belong 

 to widely separated parts of a natural system, but such cases are not 

 common, and in such instances, closer observation furnishes some clue 

 to the discovery of their real affinities. 



A Gasteropod Mollusk has a soft elongated body with a calyptriform 

 mantle on which the shell is moulded. Where the cone is short and 

 wide below the shell is nearly or quite simple and limpet-like; where 

 it is high and not very wide at the base, it is usually spirally twisted, 

 so that the shell is turbinated. Monstrosities of some of the snails 

 occur in which the spire is drawn out with only a slight spiral twist, and 

 the normal condition of the Wentletrnp (Scalaria pretiosa) shows the 

 successive whorls prevented from touching and uniting, so that we see 

 their separation, though the convoluted form is perfect. From these 

 observations we may trace the relation between the most elongated 

 spiral and the simplest expanded cap, and as it is obvious that the 

 same deviations in this respect may occur in families otherwise differently 

 constructed, we understand the phenomenon of a water snail resembling 

 a limpet, and a Sigaretus, a Venus's ear, whilst the limpet-like snail 

 occurring in fresh waters and being pulmobranchiate, and the Sigaretus 



