J. W. Judd — On tli£ Shell-groivth of Fossil Oysters. 355 



beneficent agency still in operation — first, in conserving' our coal 

 strata by lowering them so as to admit of deposition, to a vast 

 amount, taking place above them ; and again in uplifting them so 

 as to bring them, and indeed every other economic stratum of our 

 island, within easy access, to^ meet our varied and ever-increasiag 

 needs. 



II. — On the Ajs^omalous Mode of Growth of Certain Tossie. 



Oysters. 



By John W. Judd, F.G.S.. 



(PLATE IX..) 



IN that rich storehouse of fossils, the Cornbrash, there occur, as all 

 geologists who have been in the habit of collecting from it are 

 aware, several species of the genus Ostrea, two of which are large 

 and conspicuous shells. One of these is the everywhere abundant 

 and very characteristic plicated form known as 0. Marshii, Sow. 

 {0. flabelliformis, Lamk. ?) ; the other, which is far less common, 

 but by no means rare, is flat and smooth, and sometimes considerably 

 elongated in form. This latter species does not appear to have 

 received a name in this country,, but it may not improbably be 

 identical with the 0. exarata of Goldfuss (Petrifacta Germanise^ 

 Theil ii. z. 5, tab. 72, fig. 9), a shell first obtained from the TJnter- 

 ooliih of Grafinberg. 



If we look over any collection of specimens of this Cornbrash 

 Oyster, we shall almost certainly be struck with the appearance of 

 some detached upper valves, which exhibit on their outer surfaces 

 markings very different from those belonging to any known 

 species of Oyster, and unmistakably resembling those which 

 characterize totally different classes of shells. The interiors of 

 these valves, however, exhibit no peculiarity whatever, but have 

 the- smooth surface with the impression of the single adductor 

 muscle, which characterizes that group of the MoUusca to which the 

 Oysters belong. Occasionally the markings on the outer surfaces of 

 these oysters appear to be nearly as distinct and sharply defined as 

 those of the shells which they imitate. The shell of which the 

 markings appear to be most frequently copied in this manner is a 

 clavellate Trigonia, itself a tolerably, abundant fossil of the Corn- 

 brash, and known as T. Scarburgensis, Lycett ; but specimens of the 

 oyster bearing the markings of Ammonites Herveyi, Sow., and other 

 common Cornbrash shells, are by no means rare. 



Of course the first suggestion which, offers itself to the mind, in 

 attempting to account for these appearances, is that the oysters are 

 merely impressed with the features of the objects to which they 

 happened to grow attached. But a moment's consideration suggests 

 a fatal objection to this explanation ; we are required to account for 

 the production, not of a cast, but of a facsimile of the shell copied. 

 In the case of a remarkable oyster in the British Museum, which 

 bears the markings on the back of an Ammonite of the group of the 



