10 INTHODUCTIOX TO COXCHOLOGY. 



in organic development to that internal frame- work which 

 pertains to the higher orders of vertebrate animals^ each 

 kind has its allotted period and uniformity of growth^ and 

 exhibits certain characters and impressions equally indicative 

 of peculiar structure, and suggestive of subsidiary characters 

 for the distinction of groups. 



The Cov:ry shell may be known by its highly poKshed 

 and porcellaneous surface to be more or less enveloped by 

 some appendage of the animal inhabitant ; the StpJwnaria 

 ofiPers indications of a siphon; the Venus reveals the form 

 assigned to the lobes of the mantle; and who may observe 

 the series of holes that embellish the Haliotk without con- 

 jecturing that they answer some important purpose in the 

 animal economy ? And his conjecture would be right ; for 

 they are perforated, at different periods, for the passage of 

 organs conveying water to the breathing apparatus. 



The same obvious reference to beauty of effect, which 

 gave rise to an arrangement of little prisms for refracting 

 rays of light, and thus producing a variety of iridescent 

 hues, is equally apparent in the configm-ation of different 

 shells. This pleasing fact is strikingly exemplified in the 

 spiral form of many of these elegant receptacles. Bound 

 or oblong cases would effectually defend the inhabitants 



