21 



meeting with specimens of the oyster wherein the outside of both 

 shells is strongly stamped with a mark as if they had been sealed 

 with some regular figure of the diaper fashion. You have the two 

 shells. You think you have all that is needed ; and I have often 

 been amused to see how collectors have been puzzled to make out 

 what this mark can be. The explanation is, in fact, as plain as 

 possible. There exists, in the chalk, a very remarkable group of 

 fossils, called the Ventriculidee, distinguished by a structure which 

 was only discovered a few years ago, and the discovery of which 

 showed that mathematical forms are not unknown in animal life, as 

 had been before supposed.* Though these fossils are, as we find 

 them, very delicate, it lias been proved that, when living, their forms 

 were not only durable, but particularly firm. Now, the oysters, 

 — many of which were, in truth, but parasites, — were very fond of 

 settling their young on these Yentriculidse. Some of the oysters, 

 after making a small fixed bottom of shell, throw up a rising wall ; 

 — others creep, so to speak, with their shell along the surface of 

 whatsoever it may be that they are first fixed to. Those of the 

 latter that are fixed on the sides of the Ventriculidse, fasten them- 

 selves with closeness to its shape, and follow all its indentations. 

 The new shell is formed, in both valves, just beyond the edge of the 

 animal within, so that both valves — which are then of the finest 

 thinness — take the same impression. Though the animal afterwards 

 grows, and layer after layer is added to the shell as the animal does 

 thus grow, — so that the valves become thick, and the inner surface 

 of each valve becomes quite smooth, — the outer surface of the shell 

 bears for ever, on each valve, the stamp which was first thus got. 



The last example shows that even an actual fossil often cannot be 

 understood unless the nature of others be also understood. But it 

 is no less certain that a mere mass of rock may itself be directly 

 suggestive, and may open to the observer glimpses of something be- 

 yond what he yet knows ; — thus still teaching that each fact that can 

 be found is related to facts beyond its mere self. Let me take one 

 example to illustrate this. At the bottom of the London Clay you 

 find what are called " Plumpudding Stones;" — which are, indeed, 

 amazingly like plum -pud dings, with the small difference that they 



* See " The Ventriculidte of the Chalk ; their Microscopic structure, affinities, and 

 classification," etc., by Toulmin Smith. 1848. 



