6 



MOLLUSCA. 



Ammonites. 



principal fokls ; aperture approacliing to six-sided, equal to 

 about two-fifths of the diameter, and one side embracing the 

 adjoining volution ; septa numerous and very foliaceous. 



First discovered by G. A. Mantell, Esq. at Ringmer, east 

 of Lewes, Sussex. 



Mr Mantell says, " The number and disposition of the ribs 

 and tubercles of this species are so various, that although it 

 is one of the most abundant productions of the Gray Marie, 

 its specific characters are not easily defined. 



The general form of the shell is discoidal, the volutions 

 (which, when perfect, arc nearly cylindrical) being flattened 

 by compression, as in the specimens figured by Mr Sowerby. 

 The inner wreaths in those which are compressed are nearly 

 two-thirds concealed, but in more perfect examples are less 

 deeply inserted. The costse are round, and extend alter- 

 nately across the whorls, the intermediate ones embracing 

 about two-thirds of the volutions. Tlie tubercles constitute 

 the following varieties : — 



" Variety l.coitata With two rows of tubercles, tablet 



21, fig. 9. Two tubercles are placed on every rib, and form 

 a row on each margin of the ambit or back of the shell. This 

 is a beautiful cast, from Middleham. The specimen, fig. 1, 

 tablet 22, also belongs to this variety. It exhibits the folia- 

 ceous septa, and the situation of the siphunculus. It was 

 collected by my friend, Thomas Woolgar, Esq. of Lewes." 



We have represented this variety on pi. IV. figs. 4 and 9. 



" Variety 2. tuherculo-costata. — With six rows of tubercles. 

 This variety, in addition to the marginal tubercles, has four 

 rows, which are placed on the lower costce only, each side of 

 the shell having one set on the margin of the umbilicus, and 

 another at a short distance above it. 



Variety 3. tuberctdata. — With eight rows of tubercles. 

 The two additional sets which distinguish this variety are 

 placed on each side, midway between the margin of the 

 ambit and the second row of tubercles from the umbilicus. 

 These intermediate tubercles occur on every rib, each of the 

 larger costae being ornamented with eight, while the shorter 

 ones have but four. From the numerous tubercular projec- 

 tions on this vai'iety, the outer volution is somewhat penta- 

 gonal. 



The septa of Ammonites Mantelli are numerous, and very 

 foliaceous. The form of the aperture varies in diflrerent 

 specimens, but its width is in general equal to about two- 

 fifths of the diameter of the shell. The siphunculus is small, 

 and extends along the centre of the ambit. 



This species frequently attains a large size, exceeding one 

 foot and a half in diameter, but in these the tubercles are 

 nearly obliterated." 



Mantell's Ammonite has been found in almost every spot 

 in Sussex whore an excavation has been made in the Gray 

 Marie. 



7. A. cosTATUs The Ribbed Ammonite, pl.,V. fig. 2. 



With four depressed volutions ; margin three-sided, broad, 



and flattened ; volutions about two-thirds concealed, with 

 strong radiating ribs, some of which, in the inner volutions, 

 do not reach entirely across ; sides somewhat flattened ; aper- 

 ture six-sided ; ambit trigonal. 



From the Limestone at Ringmer, Sussex. 



This species is nearly allied to Ammoniles Ma?itelli, and is 

 probably only a variety of that shell. 



8. A. MiNUTUS. — The Minute Ammonite, pi. IV. fig. 10. 

 ■ Sowerby, Min. Conch. I. p. 116, pi. 53. fig. 3. 



Orbicular, with a small umbilicus, thickness and diameter 

 nearly equal ; inner volutions concealed, with numerous con- 

 centric, wide stria?, about twenty-four in number ; aperture 

 semilunar ; from two to three lines in diameter. 



Found at Folkstone, Kent, by Mr Gibbs. 



9. A. Lamberti Lambert's Ammonite, pi. V. fig. 1. 



Sowerby, Min. Conch. III. p. 73, pi. 242, figs. 1, 2, and 3. 

 Discoid, depressed, numerously radiated, curved over 



the back ; alternately long and short, but rarely furcated ; 

 the longer radii are strong, and emanate from the inner 

 margin of each volution, curving forward when past the 

 centre, at which place they sometimes branch, but generally 

 from this situation the shorter ridges take their rise, and 

 proceed to the edge, producing an imperfectly crenulated, 

 sharp carina ; aperture lanceolate. Diameter about four 

 times its thickness ; greatest diameter two inches and a half. 



In some individuals the radii are considerably more pro- 

 duced than in others, especially in the last volution of the 

 larger ones, where they become proportionally less numerous. 



Found at Weymouth, Portland Island, and Sandfoot 

 Castle. 



10. A. ACUTUS — The Acute Ammonite, pi. V. fig. 3. 

 Sowerby, Min, Conch. I. p. 51, pi. 17, fig. 1. 

 Somewhat depressed, with three or four volutions, the 



inner ones half exposed ; surface provided with slightly 

 bent ribs, which gradually thicken as they diverge from the 

 inner margin, where they commence, and terminate a little 

 way beyond the centre of the volutions ; slightly earinated, 

 with the margin crenulated and flattish ; aperture somewhat 

 cordiform, and two-fifths the diameter of the shell. Diameter 

 an inch and three-eighths ; thickness three-eighths. 



Found in the Clifl^, near Minster, Isle of Shepey, and in 

 Portland Island, and the London Clay. 



11. A. OMPHALOiDEs. — The Umbilicatcd Ammonite, pi. 

 V. fig. 4. 



Sowerby, Min. Conch. III. p. 74, pi. 242, fig. 5. 



Gibbous, inner volutions half concealed, the outer ones 

 increasing rapidly ; with produced, waved ribs, bending 

 forward in the centre of the back, and several of which are 

 furcated, but not alwaj's united to the larger ones ; back 

 broad and rounded ; aperture transversely oblong, occupying 

 more than half of the diameter of the shell. 



Found near Wej'mouth, and in Portland Island. 



Sowerby says the ribs sometimes unite to two alternate 

 ones on opposite sides of the volutions, forming a zig-zag line 

 upon the back. 



12. A. QUADRATDs. — The Square-mouthed Ammonite, 

 pi. V. fig. 5. 



Sowerby, Min. Conch. L p. 52, pi. 17, fig. 3. 



Somewhat depressed, with four or five volutions, the inner 

 ones half concealed ; surface covered with produced, undu- 

 lating, nearly uniformly thick, furcated ribs, extending into 

 the earinated and crenated margin, which is not flattened, 

 with irregular intermediate shorter ribs hardly reaching 

 the centre ; aperture somewhat quadrangular, extending to 

 about a third of the diameter of the shell. Diameter an inch 

 and five-eighths ; thickness half an inch. 



Found in a gravel pit at Brandstone, near Framlingham, 

 Suff-olk. 



13. A. GiGANTEUs. — The Gigantic Ammonite, pi. V. fig. 6. 

 De Montfort, p. 92 ; Lister, pi. I04G ; Sowerby, Min. 



Conch. IL p. 55, pi. 126. 



