TESTACEA. 21 



are sufficiently delicate and symmetrical, and of some interest, when we 

 can discover the animals inhabiting them. 



It is chiefly with regard to the latter, should there be an opportu- 

 nity, that I desire to say a few words, and only as illustrating general 

 principles. 



The first 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 

 there is some ambiguity whether it is totally displayed or otherwise. 



SECTION I.— UNI VALVES. 



Nature seems to advance by slight transitions through the great 

 field of creation. If any great hiatus appears which cannot be occupied 

 in the present state of knowledge, the animals originally preserving its 

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

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

 has occurred ; but because something leading to the destruction of life, on 

 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 links than in others. There the living creatures forming 

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

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

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

 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 is 

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

 are various species where very little benefit is in this manner derived 

 from the shell. 



Sometimes there is so great a disproportion between the body of the 



