40 FOSSIL MOLLUSCA OF THE CHALK. 



A. testa inflatd, costatd, interdum tuberculatd : anfractihus rotundatis : costis rectis, 

 sim])licibus, incequalibus, ad dorsum continuis ; junioribus tuberculatis ; adultis avgulatis, 

 vet rotundatis : dorso rotundato, transversim costato , juniore trituberculato : umbilico parvo, 

 prof undo : aperturd semi-eUipticd : septorum lobis lateralibus, primo incequaliter bifida , 

 cceteris trifidis : variat angulis, tuberculisque costalibus jjIus, minusve conspicuis. 



Shell inflated, with few rounded or angular whorls, crossed by numerous strong, 

 unequal ribs : back round, or angular at the sides : umbilicus small and deep, with 

 half the inner whorls visible : ribs alternately longer and shorter ; the longer ribs 

 commence from a tubercle at the edge of the umbilicus, and sometimes bear a second 

 tubercle on each side of the whorl ; the shorter ribs rise on the side of the whorl, nearer 

 to the umbilicus than to the back ; all the ribs cross the back : in young shells there are 

 three tubercles on the dorsal part of each rib, which give a slight angularity to the back 

 and to the mouth ; in older shells the rib is strong and prominent over the back, without 

 any tubercles, and the back and the mouth are rounded : the superior lateral lobe of the 

 septa is unequally bifid, the two others are trifid ; only two lateral lobes are visible on the 

 side of the whorl, the second auxiliary lateral lobe being on the slope of the umbilicus. 



Diameter, 4^ inches ; height of last whorl, 2 inches ; width of aperture, 2 inches. 



Common in the Lower Chalk of the South of England. 



This species was first described by Dr. Mantell from a very imperfect specimen, but 

 w-as afterwards admirably figured in the ' Mineral Conchology ■/ nevertheless it was united 

 to A. Manielli by D'Orbigny, Broun, and other Continental writers. They are, however, 

 to be distinguished when young by the row of tubercles on the middle of the back of 

 A. navicularis, and when old by the ribs of that species continuing elevated over the 

 middle of the back j while in A. Mantelli there is a depression on the middle of the back 

 at all periods of its growth. A. Mantelli is usually flatter, more angular, and more 

 tuberculated than A. navicularis, but there are varieties which differ in these respects. 

 The species are both found in the Lower Chalk; but I have seen no specimens of A. navi- 

 cularis from the Upper Green Sand. In the ' Prodrome de Paleontologie,' M. D'Orbigny 

 has again separated the two species. 



A. Gentoni of Defrance and Brongniart, is the young of A. navicularis, in the con- 

 dition of my figure 5 c. 



A. Milletianus of Dixon (not of D'Orbigny), is one of the many forms of this species, 

 nearly the same as my figure 5. 



27. Ammonites Mantelli, Sowerbi/. Plate XVIII, figs. 4 — 7. 



Ammonites Mantelli, Sotcerby. Min. Conch., t. Iv. 



— — Mantell. Fossils of the South Downs, t. xxi, fig. 9 ; t, xxii, fig. 1 . 



— — D'Orbigny. Paleont. Fran9. Terr. Cret., t. 104, figs. 1 and 2. (Not 

 ^ t. 103.) 



— — Id, Prodrome, vol. ii, p. 146. 



