CEPHALOPODA. 13 



volutions, the last formed partly concealing the previous ones, with about thirty to forty 

 rounded or obtuse and nearly straight ribs on the inner margin, which generally become 

 bi- or trifurcate about the middle of the volution, and some pass over the back, giving it 

 a costated appearance; the ribs, however, are not aways confluent, an intermediate one 

 frequently arising about the middle of the volution ; from the manner in which each 

 volution is enveloped, the previous ones only exhibit the simple costse, as seen in the 

 specimen figured at Tab. I, fig. 3. The aperture is semi-ovate and compressed. The 

 sinuosities of the septa are not to be traced with any degree of accuracy, but they appear 

 generally to resemble those indicated by D'Orbigny. ' Terr. Jurass.,' t. 148. {Am. Bakeria.) 



Differing as our figure 1 does from that given by Prof. Buckman in the * Geology of 

 Cheltenham,' we have no doubt of the identity of the specimens, having been enabled, 

 through the kindness of that gentleman, to compare the original form. All the 

 specimens we have examined of this species are more or less compressed, and this cause 

 may have partly influenced the peculiar saggitate form of the aperture in the individual 

 shell selected by Prof. Buckman for illustration. 



The costae which ornament this shell in the young state, and for a considerable period 

 of its growth, become partially obsolete in a more advanced stage. Perfect specimens of 

 this shell, showing the fact, are excessively rare, but we have collected large fragments of 

 this species on Sevenhampton common, in which the character is clearly exhibited. 



Locality. The specimen figured is in the colleetion of James Baber, Esq. F.G.S., and 

 was obtained from the Stonesfield slate of Stonesfield. It also occurs in the same 

 formation at Sevenhampton common. 



Ammonites Waterhousei. Plate I, figs. 4, 4<z. 



Ammonites discus, D'Orb. Terr. Jurass., p. 394, t. 131. 

 — — D'Orb. Prod. Paleont., p. 296. 1850. 



A. Testa compressd, sub-carinatd, anfractibus compressis, latis, externe angidatis; 

 Icevigatis ; dorso acuto ; umbilico angustato ; aperturd sagittatd. 



A compressed, discoidal shell, formed of very compressed and nearly embracing 

 volutions ; the inner half of the shell flattened and smooth ; the outer, with obtuse, rather 

 distant and flexuous costse, terminating near the margin ; keel acute ; mouth sagittate. 



This specimen, from the Great Oolite of Minchinhampton, agrees in all the characters 

 with the Am. discus, figured and described by M. D'Orbigny, 'Terr. Jurass.,' pi. 131, and 



1 Unfortunately the specimen figured illustrates only the young state or inner volutions of this species, 

 not having received at that time the finer specimens belonging to Professor Buckman, and from which the 

 following description was drawn up : "A. gracilis. Keel crenated ; volutions six or seven, half concealed ; 

 ribs straight, passing over the back, and thus making the crenations of the keel ; sometimes two or three 

 ribs are confluent towards the front ; diameter about 9 inches ; thickness 1 \ inches ; aperture sagittate." 

 (Geol. of Chelt., p. 104.) 



3 



