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 trifureate 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 costa, 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. (4m. Bakerie.) 
Differmg as our figure’ 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 costes 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 collection 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 Warrrnoussl. Plate I, figs. 4, 4a. 
AmMonitEs Discus, D’Oré. Terr. Jurass., p. 394, t. 131. 
— — JD Orb. Prod. Paléont., p. 296. 1850. 
A. Testa compressd, sub-carinatd, anfractibus compressis, latis, externe angulatis ; 
levigatis ; dorso acuto ; umbilico angustato ; apertura sagittata. 
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 coste, 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.,’ pl. 131, and 
! 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: ‘‘ 4. 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.) 
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