26 FOSSIL MOLLUSCA OF THE CHALK. 



of the back ; umbilicus small, with steep sides ; whorls more than half concealed ; aper- 

 ture oval ; septa with three slightly indented lateral lobes. 



Diameter, IJ inch; width, fths of an inch. 



Only one specimen has been seen, which was presented by Mr. E. H. Bunbury to the 

 Museum of Practical Geology ; it is from the Chalk with sihceous grains of Chardstock, 

 Somersetshii'e. The species is named after its discoverer. 



8. Ammonites peramplus, Mantell. Plate X, figs. 1 — 3. 



Ammonites pebamplus, Mantell. Fossils of the South Downs, p. 200. 



— — Sowerby. Min. Conch., t. 357. 



— — D'Orbigny. Paleont. Fran9. Terr. Cret., t. 100, figs. 1, 2. 



— Peosperianus, lyOrbigny. Loc. cit., t. 100, figs. 3 and 4, 



A. testa suhinjlatd : anfractibits rotundatis ; juniorihtis costatis, costis JtexuosiSy 

 ampledentihus, quarum sex majoribtis intus tuberculatis ; adolescentibtcs utrinque undato- 

 costatis, dorso piano ; adultis rotundatis, intus sub-undatis ; senioribus rotundatis, simplici- 

 bus : aperturd imprimis semi-circidari, delude ovatd. 



Shell gibbose, with rounded whorls, of which nearly two thirds are concealed, varying 

 in proportions and ornament at different periods of growth ; side of the umbilicus very 

 steep ; septa with four or five unequally trifid, branching lateral lobes. 



The young shell, up to a diameter of two inches, has the whorls broader than they are 

 high, ornamented by about twenty-five rounded ribs, which bend forward as they cross the 

 back ; of these six are stronger than the others, and rise from six large pointed tubercles 

 on the edge of the umbilicus, fig. 3, A. prosperianus, D'Orb. At a diameter of four to 

 six inches, the intermediate minor ribs disappear, the tubercles become obsolete, and the 

 back smooth, and there remain about twelve broad, rounded, straight ribs on each side of 

 the whorl, which do not meet on the back, fig. 2. 



When the shells reaches a diameter of a foot, the whorl is higher than broad, and 

 nearly smooth, with no trace of its previous ornament, except a series of blunt ill-defined 

 tubercles round the umbilicus, connected with faint undulations on the inner half of the 

 whorl (fig. 1). 



Very old specimens, which may be found exceeding two feet in diameter, have the 

 whorls smooth and entirely devoid of ornament, and the aperture laterally compressed. 



A. peramplus is found in the middle part of the Chalk throughout the counties of 

 Kent, Sussex, Surrey, Hertfordshire, and Wiltshire, and the Isle of Wight ; but is most 

 abundant in the South Downs. It reaches from the white chalk without flints high up into 

 the chalk with flints, but large individuals are only found in the lower parts of its range, 

 the uppermost beds in which it occurs only containing very small specimens. M. d'Orbigny 

 quotes it only from the Upper Green Sand of the South of France. 



M. d'Orbigny conjectured, p. 335, that his A. prosperianus might, perhaps, prove the 

 young of A, peramplus, which farther observation has shown to be the case. 



