II :A. LM 



are sntli.-ieiitly delicate and symmetrical, and of some interest, wln-n u- 

 discover the animals inhabiting them. 



It i< chiellv with n-ininl to the latter, should then- Ix- an opportu- 

 nity, that I dr-irc to say a few words, and only as illustrating gei, 

 principle!. 



The lit^t step is to discover their precise figure, a measure of appa- 

 rent facility, but by no means so in reality, for it is seldom completely 

 displayed. Moreover, the whole is momentarily so changeable, that 

 th.-re is some ambiguity whether it is totally displayed or otherwise. 



SECTION I. UNI VALVES. 



Nature seems to advance by slight transitions through the great 

 Ik-Id of creation. If any great hintnx appears which cannot be occupied 

 in the present -tate of knowledge, the animals originally preserving it* 

 continuity have probably become extinct with the lapse of time. It is 

 not simply because years have rolled after years, that such u condition 

 has occurred ; but because something leading to the destruction of Hi . 

 the cessation of evolution, has intervened in their course. 



But the operation of such causes has not been equal throughout. 

 In some countries the separate genera seem more definitely connected by 

 intermediate link." than in others. There the living creatures forming 

 the chain, or part of them, still subsist. In Britain the interruptions art- 

 many. I doubt not tint the researches of modern naturalists will tend to 

 their diminution, and restore a portion of those legions swept away \>\ 

 time or pestilence. Meanwhile, organic remains testify how many have 

 perished. 



We generally assume, and with much appearance of truth, that the 

 shell is specially designed for the protection of the tenant which i- 

 received into its cavity, and thus sheltered from injury. However, then- 

 are various - \h"re very little benefit is in this manner d. : 



from the shell. 



Sometimes there is so great a disproportion ;i the body of the 



