366 J. W. Jucld — On the Shell-growth of Fossil Oysters. 



Ligati, an attempt to meet this difficulty has been made by the 

 suggestion that the oyster had grown on the inside of a dead 

 Ammonite shell. Setting aside the inherent improbability of this 

 explanation, it is evident that it will not serve us in the case of 

 oysters taking the markings of Trigonia, as the inner surfaces of 

 these latter bear no resemblance whatever to their outer. 



So remarkable is the appearance of shells of one genus bearing 

 the superficial markings of those of a totally different group, but 

 otherwise retaining all their essential characters, that I was at one 

 time even tempted to speculate whether we might not have here in 

 the Oolitic rocks an actual case of mimicry, possibly for protective 

 purposes, similar to those which obtain so frequently among many 

 living animals. The collection and careful examination of a large 

 series of specimens, while it has dispelled this idea, has led to the 

 true explanation of the phenomenon — an explanation which is not 

 a little curious and interesting. 



Among the thin-shelled Ostreidce (Anomia, Placunopsis, Placuna, 

 Plicahda, etc.), we frequently find specimens that have grown 

 attached to shells or other foreign bodies in which, owing to its 

 tenuity, the whole organism, lower valve, animal and upper valve, 

 has become moulded on to and taken the markings of the surface on 

 which it lies. In Plate IX. two examples illustrating this are 

 shown. Fig. la, lb, represents the upper and under sides respec- 

 tively of a specimen of Placunopsis Jurensis, Mor. and Lye, which 

 has grown attached to the side of Goniomya V-scripta, Sow. This 

 specimen, which was obtained from the Cornbrash of Scarborough, 

 by the late Mr. Bean, is now in the British Museum. In Fig. 2 is 

 shown a still more curious specimen ; it was obtained by Dr. Lycett 

 from the Inferior Oolite of Eodborough Hill, near Stroud, and is in 

 the Museum of Practical Greology, Jermyn Street. This specimen is 

 a Placunopsis, of a species not yet described, which has grown upon 

 a clavellate Trigonia; the surface of its upper valve exhibits the 

 peculiar markings of the Placunopsis in combination with those of 

 the Trigonia. 



If we were to find the upper valve of a Placunopsis, like either of 

 those we have been describing, lying detached in the rock (which 

 we seldom or never do), it would resemble, in the character of its 

 outer surface, the oysters to which we have referred ; but its inner 

 surface would differ altogether by following all the foldings and 

 wrinklings of the outer, instead of being perfectly smooth. 



Occasionally we find specimens of our Cornbrash oyster with both 

 valves preserved. A good example of this is shown in Plate IX., 

 Fig. 3. The specimen which is in the British Museum was collected 

 by W. Cunnington, Esq., at Weymouth. 3a shows the upper 

 valve displaying very distinctly the characteristic tubercles of the 

 Trio-onia ; 3&, the lower valve, in which precisely the same markings 

 are exhibited in reverse or as a cast. On separating the two valves, 

 we find the interiors quite smooth, and exhibiting no trace whatever 

 of the prominent markings of the exterior. 



If we consider the characters presented by this specimen, at the 



