J. W. Judd — On the Shell-growth of Fossil Oysters. 357 



same time bearing in mind the features presented by the attached 

 thin-shelled Ostreidce, like Placunopsis, and also the mode of 

 growth of the shell in the oysters by the deposition of successive 

 laminse from the mantle of the animal, we are led to realize the exact 

 manner in which the singular examples to which we have referred 

 have been produced. 



This we have illustrated by the ideal section, Fig. 4, which repre- 

 sents a Trigonia shell (a), upon which an oyster (b) has grown. At 

 an early stage of its growth the valves of the oyster would be repre- 

 sented by 1-1, l''-l'' ; these being then nearly in apposition, and the 

 animal like them extremely thin, and partaking of all the undula- 

 tions of the surface of the Trigonia. In this state the oyster would 

 exactly resemble the specimen of Placunopsis represented in Fig. 2. 

 The subsequent growth of the oyster was almost entirely confined to 

 an increase of the thickness of the shell. The two first-formed 

 laminse, 1-1, I'-l', would not be in any way altered by the shell's 

 subsequent growth, though they would be thrust gradually farther 

 and farther apart. The successively deposited laj^ers of shelly 

 matter would naturally be less and less conformed to these exterior 

 laminse, and thus these markings becoming continually fainter on the 

 interior of the shell, would at last be obliterated altogether. The 

 successive stages of growth of the two valves are represented in the 

 diagram by the lines 2-2, 2'-2^; 3-3, 3^-3'; 4-4, 4^-4'; 5-5, 5^-5'; 

 and 6-6, &—6^. Of course, as the animal increases in thickness as 

 well as the shell, an excessive deposition of shelly matter will have 

 to take place at the edges of the valves, and this is always observed 

 to be the case in the specimens of which we are speaking. 



That this is the true explanation of the remarkable appearances to 

 which we have directed attention in this note is confirmed by an 

 examination of sections of these oysters, and also by the successive 

 removal (by the aid of dilute acid) of the several laminse of which 

 the shells are composed. 



It is evident that the oysters which present these mimetic mark- 

 ings over nearly the whole of their surfaces must, in their early 

 stage of growth, have increased very slowly vertically, but rapidly 

 laterally, like Placunopsis, so that the shell must have attained 

 almost its full diameter before it began to increase greatly in 

 thickness. 



As an example of the singular objects which are sometimes pro- 

 duced in consequence of the peculiar mode of growth of these oysters, 

 we have figured a specimen obtained by the late Dr. Porter from the 

 Cornbrash, near Peterborough. In this case two oysters have grown 

 side by side on the back of a large example of Ammonites Herveyi, 

 Sow., and at last have so pressed upon each other that they have 

 become united at their edges. The curvature of the Ammonite shell, 

 with the markings of its surface, have been reproduced on this double 

 specimen, the ribs of the former appearing as casts on its under side 

 (56) and in relief on its upper side (5a) ; 5c represents a section of 

 this singular specimen. 



Besides the species to which we have been referring, other oysters 



