328 



THE GEOLOGIST- 



ON AN AMMONITE WITH ITS OPERCULUM IN SITU. 



By S. P, Woodward, F.G.S. 



Opercula of Ammonites are common in ma^ny localities, especially 

 in banks and sections of tlie Kimmeridge Clay ; but they nsiially occur 

 in broken fragments, and very rarely with tbeir valves paired, unless 

 sheltered within the last whirl of the shell to which they belonged. 

 Even when thus protected the valves are generally displaced, as 

 might be expected if we consider how slight is their nnion along the 

 sntm-e, and how great were the chances of being shifted by the con- 

 traction of the animal after death, by the pressure of external mud. 



The British Museum contains several examples of Ammonites Jason, 

 A. Brightii, A. fiuctuosus, A. Ungulatus, and other species with their 

 opercula more or less shifted ; and Mr. Charles Moore, of Bath, has 

 several small shells of Ammonites ]3lanof'bis from the Lower Lias, 

 with the opercula remaining in their true position ; the smallest in- 

 dividual is only one quarter of an inch in diameter. 



I have recently obtained a specimen of Am- 

 monites subradiattis (J, Sby.) from Mr. Joseph 

 Wood, an experienced collector (formerly of 

 Bath, but now living at 23, New Union-street, 

 Moorfields), who discovered it in the Inferior 

 Oolite, of Dundry, near Bristol, with the oper- 

 culum remaining in its natural position as 

 represented in the accompanying figure. 



The shell measures sixteen lines by twelve 

 and a half, with a maximum thickness of four and 

 a half lines. It agrees with the ordinary run of 

 specimens from Dundry, and differs from the Ammonites suhradiatus, 

 example figured by Sowerby in being less com- the opercnium 



pressed, and more widely umbilicated ; the umbi- 

 licus measures four lines across, and is bordered by a steep margin. 



The operculum is flat in the middle, vAth a slight frirrow^ along 

 the suture, and is much bent down at the hinder comers where it 

 abuts against the inner whirl of the shell. It is six lines long and 

 four wide, and is sculptured externally with about twelve angulat 

 concentric furrows ; the inner surface is smooth, as shewn by the 

 fracture and removal of a portion. It closely resembles the opercula 

 of Ammwuites BrigJifii and A. lingttlatus, to which A. suhradiatus is 

 nearly related. 



Jh'ifit^h Museum, August, 1860. 



