4 SUPPLEMENT TO GREAT OOLITE MOLLUSCA. 



diameter, the aperture having a height and breadth of two inches ; its more advanced 

 growth will account for the difference of figure when compared with those of D'Orbigny, 

 Quenstedt, and of Kudernatsch ; but in truth, the variability of figure extends not less 

 to individuals than to the stages of growth, for in no instance does there appear to be a 

 very near agreement of figure. 



Geological Position and Localities. The sole specimen in my collection is from the 

 Great Oolite, near Tiltups Inn, two miles south of Nailsworth ; another specimen, 

 apparently from the same locality, is in the collection of my friend, Dr. Wright, of Chelten- 

 ham. The foreign localities are St. Maixent, Deux-Sevres ; Masigny, Vendee; Nantua, 

 Ain ; Vezelay, Yonne ; Wohnkammer, Swinitza. 



AMMONITES DISCUS, Sow. Tab. XLI, fig. 8, 8 a. 



NAUTILUS DISCUS, Sou-. Min. Con., 1813, i, tab. 12. 



AMMONITES DISCUS, Sow. Ibid., 1815, Suppl. Ind. to rol. i, p. fl. 



Morris. Catal., 1854, p. 291. 



Oppel. Juraformation, p. 472. 



Testa discoidea, angusto umbilicato, dorse ant/usto acute carinatis, latcri/nis exfcrne, 

 valde comprcssis, Icevigatis ; apertura sayittcpformi. j'Etate juniori lateribus costis dis- 

 tantibus flexuosis. 



Shell discoidal, with a narrow and deep umbilical cavity, the back acutely keeled; the 

 sides of the volutions near to the back are much flattened and smooth ; the aperture is 

 sagittate, the margin of the umbilicus is rounded. In the young state, when the diameter 

 does not exceed three inches, the sides are ornamented with regular distant, depressed, 

 flexuose costa3. 



The lobes are comparatively simple, with few ramifications, and have but little depth ; 

 the saddles are in a corresponding manner but little produced ; they therefore differ 

 altogether from the septa of A. discus, D'Orbigny, and from the A. sub-discus, of the same 

 author; they are, however, more complicated than is seen in A. discu*, Quenst. 

 (' CEPHALOPODEN,' tab. viii, fig. 13); A. Stau/ensis, Oppell, from the inferior Oolite of 

 Boll, Balinger, &c. They also differ from the description given by Roemer ('Nord. 

 Ool.,' p. 190) of an Ammonite attributed by him to A. discus, Sow., from the lower Coral 

 Rag of Heersum. 



The general figure is less discoidal than A. Waterliousei, Mor. and Lye. (A. discus, 

 D'Orb.) ; it differs also from that species by the absence of the flattening upon the inner 

 portion of the sides of the volutions. From A. sub-discus, D'Orb., the general figure 

 differs in the more acute back and in the smaller umbilicus. 



The specimen figured in the ' Mineral Conchology,' is an adult shell, and smooth ; the fine 

 specimen selected for our illustration exhibits the septa, and also some traces of the 

 falciform costse proper to the young shell. I am obliged to Mr. Woodward, of the British 



