Illustrations of the fossil conchology of Great Britain and Ireland, with the description. And localities of all the species / by Captain Thomas Brown.

  • Thomas Brown
Date:
1849
Catalogue details

Licence: Public Domain Mark

Credit: Illustrations of the fossil conchology of Great Britain and Ireland, with the description. And localities of all the species / by Captain Thomas Brown. Source: Wellcome Collection.

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    which they are abruptly reflected, and imperceptibly disap¬ pear ; back provided with an obtusely rounded carina, on each side of which is a furrow. Diameter varying from a foot to twenty-one inches. First discovered by that zealous geologist, Professor Buck- land, in the Blue Lias at Bath and its vicinity. 29. A. vaeians.—The Variable Ammonite, pi. VII. figs. 3, 5, and 8. Sowerby, Min. Conch. II. p. 169, pi. 176. 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; furnished 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 which the ribs ema¬ nate, and proceeding obliquely across one-fourth of the 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 costa?; 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 unfrequeutly 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 peculiarity, three principal varieties were observable, each passing insensibly into the other. ” Variety 1. 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 without the aid of numerous examples, their relation to the tubercular variety could not have been ascertained. Variety 2. intermedia. Mantel], 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,the external margin tuberculated, 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. Sowerby, Mm. 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, affords an excellent example. The inner rows of tubercles are almost efface ', 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 few 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 costae ; 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 Clav at St. Neotts, Huntingdon- shire, by John and Philip Duncan, Esqrs. in honour of whom it was named by Sowerby. 31. A. Conybeam.—Conybear’s Ammonite, pi. VII. fig. 6. A. Conybeari. 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, which 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. Varying 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. VII. fig. 7. A. planicosta. Sowei-by, Min. Conch. I. p. 167, pi. 73. Compressed, with six or eight exposed volutions, crossed by numerous obcuse, 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, mear Ilchester, at Yeovil and Evershot, Somersetshire. This stone is frequently cut into large slabs for table tops, tfcc. and when polished has a beautiful effect from the irregular order in which these numerous specimens of Ammonites planicostatus present themselves. The limestone is of a dark gray colour, and the Ammonites are dark bn w.i, or different shades of buff, and sometimes exhibiting a splended iridescent lustre. The divisions of the clumbers being tilled with crystal izrl cr- bouate of iron, adds a beautiful variety to the tints. This
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