232 INTRODUCTION TO CONCHOLOGY. 



No rock is like this Eock of ours. 



Oh ! then, if you have learned your powers 



By a just rule to estimate ; 



If justly you can calculate 



How great your need, your strength how frail, 



How prone your hest resolves to fail ; 



"When humble caution bids you fear 



A moment of temptation near. 



Let wakeful memory recur 



To this your simple monitor. 



And wisely shun the trial's shock, 



By clinging closer to your Eock." — Wordsworth. 



Little is known with certainty respecting the pecuhar 

 habits of the Patellce, or the purposes for which they are 

 designed. They are placed on the boundary line between 

 such shells as are furnished with a regular spire^ and those 

 which have none ; and they afford^ in common with innu- 

 merable mollusksj impressive instances that the Most High, 

 who has adorned this beauteous world with a variety of 

 singularly organized beings, has so admirably adjusted them, 

 that every part of the vast creation constitutes one beautiful 

 and perfect whole. To this splendid superstructure nothing 

 can be added, neither anything taken from it, without 

 producing a chasm in creation, which, however imperceptible 

 to us, would materially affect the general harmony of nature. 



