Ammonites. 



MOLLUSCA. 



9 



whicli they are abruptly reflected, and imperceptibly disap- 

 pear ; back provided witii an obtusely rounded carina, on 

 each side of which is a furrow. Diameter varying from a 

 foot to twentv-one inches. 



First discovered b^' tliat zealous geologist, Professor Buck- 

 land, in the Blue Lias at Bath and its vicinity. 



29. A. VARiAXS. — The Variable Ammonite, pi. VII. figs. 

 3, 5, and 8. 



Sowerby, Min. Conch. II. p. 169, pi- 1"6. Mantell, Geo- 

 logy of Sussex, p. 115, pi. 21, figs. 2, 5, and 7- 



Discoidal, compressed, rather thick, subumbilicate, carinated ; 

 with three or four half inserted volutions ; furnislied with 

 transverse, bifurcated, undulated ribs, studded with from six 

 to eight rows of somewhat obtuse tubercles ; carina acute, 

 entire ; aperture sagittate ; siphuncle supposed to be external. 

 The umbilicus is shallow, and the sides smooth, bor- 

 dered by a row of small tubercles, from whicli the ribs ema- 

 nate, and proceeding obliquely across one-fourtii of tlie 

 volutions, rise in the form of tubercles, and then diverge into 

 two branches, all of which terminate in a tubercle on the 

 exterior margin ; the keel is smooth, prominent, and acute ; 

 each margin furnished with a series of opposite tubercles. 



This species is one of the most proteiform of the Ammo- 

 nites, and subject to great variety in the form, disposition, 

 and number of the tubercles and costae ; but its acute, entire 

 carina, in connection with the tubercular, bifurcated ribs, at 

 once distinguish it. 



In size this fossil varies from an inch to six inches in 

 diameter, and is not unfrequently compressed into an ellipsis 

 or cordiform shape. No specimen has been yet found with 

 more than four volutions. 



Mr Mantell says, " In a suite of fifty specimens, in which 

 every individual presented some peculiaritj^ three principal 

 varieties were observable, each passing insensibly into the 

 other." 



Variety ]. subplana, pi. VII. fig. 8. 



Mantell, Geology of Sussex, pi. 21, fig. 2. 



" The volutions depressed, radii linear, inner row of 

 tubercles obscure, external margin crenated, keel but slightly 

 elevated, aperture sagittate. 



Some specimens of this variety are nearly smooth, and the 

 keel so much compressed, that \yithout the aid of numerous 

 examples, their relation to the tubercular variety could not 

 have been ascertained. 



Variety 2. intermedia. 



Mantell, Geology of Sussex, pi. 21, fig. 7, 8. 



The volutions in this variety are rather depressed, the ribs 

 broad and well defined, the tubercles small, and distinctly 

 marked, tiie external margin fuberculated, the keel prominent, 

 and the aperture sagittate. 



This is the prevailing form of the species, and holds 

 an intermediate rank between the smooth and tubercular 

 varieties. 



Variety 3. tuberculata. 



Sowerb}', Min, Conch, pi. 176, figs. 1, 2, 3, 4, 6. 



Volutions subrotund ; ribs short, thick, nodulous ; tubercles 

 elongated, very prominent ; carina acute ; aperture somewhat 

 rounded in form. 



" A very beautiful variety, distinguished by its projecting 

 tubercles, of which Mr Sowerby's fig. 1, aflPords an excellent 

 example. The inner rows of tubercles are almost effaced, 

 but the marginal and intermediate sets are strongly relieved, 



and in some examples become spinous. From the thickness 

 of the volutions, the aperture is obovate." 



This fossil is plentiful at Middleham, Hamsey, and 

 Stoneham, in Sussex, and also in the upper Green-sand of 

 Wiltshire. The Gray Chalk Marl is well marked by this 

 shell, as it prevails abundantly through it. Mr Mantell 

 mentions that a tew examples have been found in the lower 

 or flinty Chalks. 



30. A. DuNCANi. — Duncan's Ammonite, pi. VII. fig. 4. 

 Sowerby, Min. Conch. II. p. 129, pi. 157. 

 Compressed, inner volutions exposed about a third, with a 



few tubercles upon their sides, the whole shell beset with 

 numerous, undulating, narrow, oblique ribs, many of which 

 are irregularly furcated, somewhat obscure on the middle of 

 the sides, and terminated on their outer extremities by 

 elongated tubercles on the terminal half of the exterior 

 volution, but button-shaped on the other half, these last, in 

 many instances, extend over two of the costce ; there is also 

 a row of tubercles on the sides of the latter half, towards the 

 centre of the volution ; ambit depressed, bounded by two 

 rows of fibuliform tubercles, which are a continuation of the 

 ribs ; aperture ovato-sagittate, and equal to about half of the 

 greatest diameter of the shell. 



The sinuated edges of the septa are sharp and distinctly 

 marked. 



Discovered in the Fen Clay at St Neotts, Huntingdon- 

 shire, by John and Philip Duncan, Esqrs. in honour of whom 

 it was named by Sowerby. 



31. A. CoNYBEARi. — Conybear's Ammonite, pi. VII. fig. 6. 

 A. Conyheari. Sowerby, Min. Conch. II. p. 70, pi. 131 ; 



Phillip's Geology of Yorkshire, II. p. 164, pi. 13, fig. 5. 



Compressed, with a large, greatly produced, entire keel, on 

 each side of which is a concave groove ; volutions eight or 

 nine, usually continuing very perfect to the centre, crossed 

 by numerous rather obtuse ribs, wliich are most prominent 

 in the centre of the volutions, and are much depressed at the 

 inner sides, a little stronger next the ambit ; inner sides of 

 the volutions somewhat flattened, and slightly angular ; 

 aperture oblong-ovate. Varjing in size from two to eighteen 

 inches. 



Found in the Lias at Bath, and in the middle of England, 

 where it is not uncommon, and in the western islands of 

 Scotland. 



32. A. PLANicosTATUs. — The Flat-ribbed Ammonite, pi. 

 Vn. fig. 7. 



A. planicosta. Sowerby, Min. Conch. I. p. 167, pi. 73. 



Compressed, with six or eight exposed volutions, crossed 

 by numerous obtuse, nearly straight ribs, which widen as they 

 approach the back, and are depressed near the ambit, inclining 

 towards the aperture ; aperture circular, slightly indented by 

 the volutions. 



Found in the indurated marly limestone, called Marston 

 Stone, which occurs at Marston Magna, near Ilchester, at 

 Yeovil and Evershot, Somersetshire. This stone is frequently 

 cut into large slabs for table-tops, &c. and when polished has 

 a beautiful effect from the irregular order in which these 

 nimierous specimens of Ammonites pla?iicostatus present 

 themselves. The limestone is of a dark gray colour, and the 

 Ammonites are dark brown, or different shades of buff, and 

 sometimes exhibiting a splendid iridescent lustre. The 

 divisions of the chambers being filled with crystallized car- 

 bonate of iron, adds a beautiful variety to the tints. Ihis 



c 



