358 J. W. Jiidd — On the Shell-growth of Fossil Oysters. 



exhibit, in a greater or less degree, a tendency to this mode of 

 growth. Conspicuous among these is the well-known 0. deltoidea, 

 Sow., of the Kimmeridge Clay. In the British Museum (Northern 

 Gallery, Eoom Y., Table Case 2) there is exhibited an interesting 

 series of these shells from Boulogne, which have grown upon dif- 

 ferent portions of several species of Ammonites. 



Other oysters, which are only attached during a short portion of 

 their existence, sometimes display the characters we have been de- 

 scribing in the portion of the shell first formed. Thus the plicated 

 0. gregaria, Sow., of the Coral Eag, sometimes exhibits on the central 

 ridge of its attached and free valves respectively, the cast and fac- 

 simile of the surface of a coral to which it has been attached. 



That the Gryphoid oysters, which normally become free at a very 

 early stage of their existence, sometimes remain attached during a 

 considerable portion of their life, is well known, and has been re- 

 ferred to by Mr. Jones in his interesting paper, " On GrypJicea incurvn 

 and its Varieties," in the Proceedings of the Cotteswold Club, vol. iii., 

 p. 81. The aberrant forms thus produced have even been mistaken 

 in some cases for distinct species of oysters. These (rrj/p^oEOB, with 

 large surfaces of attachment, sometimes strikingly exhibit the pheno- 

 menon which we have described in this note. In the British 

 Museum there are two specimens of Gryphoea dilatata, Sow., from 

 the Oxford Clay of Weymouth, each of which has grown on the out- 

 side to a Trigonia, till it has attained about one-half of its diameter. 

 The portion of the shell formed during the period of attachment 

 shows clearly the markings of the Trigonia in cast and facsimile, 

 while the interior is quite smooth. In another example we find a 

 small Trigonia shell with its outer surface still embedded in the 

 incurved beak of a Gryphaea ; on the inner surface of the attached 

 valve of this latter there are no traces of the markings of the 

 Trigonia, but these are beautifully reproduced on the exterior of the 

 free valve. 



Although Goldfuss has figured some specimens of oysters with 

 the markings of other shells, such anomalous objects are generally 

 excluded from the plates which illustrate palseontological treatises. 

 An exception is found in the magnificent " Der Jura" of Professor 

 Quenstedt, a work which is the result of the most patient, minute, 

 and exact observation, and which may be quoted as the most faithful 

 delineation of the characters of a single formation in a given area 

 with which geological science has as yet been enriched. In this 

 work we find figured under the name of 0. falcifer (Atlas zum Jura, 

 tab. 37, fig. 1) an oyster from the Upper Lias, which exhibits the 

 sigmoidal markings of the shell of Ammonites falcifer, Sow. Prof. 

 Quenstedt does not appear to have noticed the remarkable character 

 of the smooth surfaces of the anterior of these oysters ; but he 

 remarks that the animal was probably very thin, and in this respect 

 allied to that of Placuna. In the same work, tab. 6, fig. 9, tab. 11, 

 fig. 9, and tab. 23, fig. 6, there are represented other examples of 

 Ostreiform shells which have taken the markings of the shells to 

 which they have grown attached. 



