10 



MOLLUSCA. 



Ammonites. 



species also occurs in granular marly limestone, particularly 

 at Craymouth, but seldom associated with the remains of any 

 other species of shell. 



33. A. AURiTus, — The Eared Ammonite, pi. VII. fig. 9. 



A. aurifus. Sowerljy, Min. Conch. II. p. 79, pi- 134; 

 Mantell, Geology of Sussex, p. 90. 



Compressed, with four or five exposed volutions, the last 

 one large, occupying about half tlie diameter of the shell ; 

 surface covered with depressed, slightly developed, distant 

 radiating ribs, every alternate one being furnished with a 

 large obtuse tubercle, towards the inner margin of the volu- 

 tions ; exterior margin deeply grooved, and provided with a 

 series of large, obtuse, alternating, compressed tubercles, 

 projecting in tlie form of ears ; aperture oblong-ovate, slightly 

 sagittate. 



Discovered in the Micaceous Sand, rn the bed of the canal 

 at Devizes, Wiltshire, by Mr Gent ; and it has subsequently 

 been found at Ringmer in Sussex. 



34. A. SPLENDENS The Splendid Ammonite, pi. VII. 



fig. 10. 



Come d' Amman fort plate, unid et ornee de fleiirs. Bour- 

 quet Traite des Petrifactions, pi. 48, fig. 312. — Ammonites 

 splendens. Sowerby, Min. Conch. II. p. 1, pi. 103, figs. 1, 2, 3 ; 

 Mantell, Geology of Sussex, p. 89, pi. 21, figs. 13, and 17. 



Involute, compressed, provided witli tliree or four volutions, 

 the inner ones deeply inserted, being about three-fourths 

 concealed, and the outer ones rapidly increasing in dimen- 

 sions ; sides flattened, with transversely radiating, depressed, 

 close ribs, sliglitly curved towards the aperture ; a row of 

 distant, greatly elongated tubercles towards the inner margin, 

 from each of which two or three ribs emanate, and make an 

 elegant curve from the inner to the outer margin, where they 

 terminate in angular eminences, forming crenulated margins 

 on the sides of the carina, the middle of which is nearly 

 plain; dissepiments sinuated and very foliaceous siphunculus 

 situate near the inner margin ; aperture oblong, almost equal 

 in length to half the diameter of the shell, and derplj' indented 

 by l!ie inner volutions. Size varying from Iialf an inch to 

 two inches in diameter. 



This truly splendid Ammonite exhibits, on its external 

 surface, the most beautiful iridescent play of colours, some- 

 times equally vivid in lustre to the finest species of Haliotis, 

 or Ear-shell. It is found in the Pyritaeeous jMarle at Folk- 

 stone, Kent, and is common in the Blue Chalk Mario at 

 Ringmer and Laughton, in Sussex. 



This species is often found with the shell remaining, which 

 is extremely thin, and of a cream white colour. The folia- 

 ceous sutures are very conspicuous in pyritaeeous casts of 

 this shell ; these difter but little from the fossil itself, except 

 in the continuous structure, under the more prominent parts 

 of the ribs, which are somewhat more depressed. Small 

 specimens are sometimes found with the keel rounded, and 

 the volutions nearly destitute of ribs, as exhibited in plate 

 103, fig. 1, of Sowerby's Mineral Conchology ; in this con- 

 dition they might be mistaken for a distinct species. 



Mantell has figured a cast in Pyritous Marie, which shews 

 the sinuous septa ; small crystals of lime are contained in 

 cavities on the opposite side of this specimen, and pseudo- 

 morphous iron pyrites is disseminated through the mass. 



35. A. LAUTUS — The Laurel Ammonite, pi. VIII. fig. 1. 

 Ammonites lautus. Parkinson, Geological Transactions, 



V. p. 58 ; Sowerby, Min. Conch. IV. p. 3, pi. 309, figs. 1, 2, 



3, 4, 5, and 6; Mantell, Geology of Sussex, p. 9, pi. 21, 

 fig. 11. 



Discoidal, involute, compressed, with three or four two- 

 thirds inserted volutions ; back narrow and deeply channeled ; 

 sides furnished with numerous, strongly arcuated slender 

 ribs, arising in pairs from a row of oblique, elongated 

 tubercles near their inner margin, and being joined by 

 alternating, intermediate shorter ones, proceed with an 

 elegant curve to the outer margin, where tliey terminate, in 

 somewhat depressed, large, alternating tubercles, usually 

 three or four to each tubercle ; these are disposed alternately, 

 so that the edges may be characterized as serrato-tuberculate ; 

 dissepiments very foliaceous ; aperture obscurely sagittate, 

 and equal in length to half the diameter of the shell. The 

 situation of the siphunculus is unknown. 



Found at Laughton, Ringmer, and Norlington, Suffolk. 



Sowerby describes the following varieties of this fossil : — 



1*'^ Ribs long, considerably arcuated and regular. Mineral 

 Conch, pi. 319, figs. 1, -2. 



2d, Having short irregular ribs provided with large 

 tubercles near their inner ends. Min. Conch, pi. 319, fig. 6. 

 It is this variety which we have figured ; the otlier varieties, 

 if such thej' be, have not come under our observation. 



This species somewhat resembles the Ammonites dentntus, 

 pi. 14, fig. 4, but the volutions are more exposed than in that 

 fossil. It is also allied to Ammonites am it us, pi. 7, fig. 9, 

 but is distinguished by its prominent and curved ribs, by 

 the ridges on the inner volutions being less, two-thirds con- 

 cealed, and by the centre one not being so tuberculous. 



3G. A. ARMATUS The Armed Ammonite, pi. VIII. 



fig. 2. 



Ammonites armatus. Sowerb}', Min. Conch. L p. 215, 

 pi. 95. 



Involute, with six or seven entirely exposed volutions, 

 which are pressed against each other, and crossed by nume- 

 rous annular ribs, each of which is provided with two series 

 of large, short, furrowed spines, to the number of five on 

 each ; the costae, after meeting on the point of the spines, are 

 continued on the other side ; aperture obscurely four-sided ; 

 siphunclo situate near the dorsal margin of the aperture. 



Found in the Alum Clay formation at Whitby, Yorkshire; 

 the Oxford Claj', middle unci south of England, and the Lias 

 at Bath. 



In the young state, this shell is quite plain, without tlie 

 slightest appearance of ribs or spines. In a more advanced 

 condition, the ribs appear, and, when it has acquired another 

 convolution, the disk is flattened. 



This Ammonite is subject to considerable variety. 



37. A. r-LANUs The Flat Ammonite, pi. \TII. fig. 3. 



Ammottites planus. IMantell, Geology of Sussex, p. 90, 

 pi. 21, fig. 3. 



Involute, carinated, compressed, deeply inserted, almost 

 smooth ; volutions crossed by nearly obsolete strioe ; keel 

 flat, with its margin crenulated ; aperture sagittate ; dissepi- 

 ments sinuate ; situation of the siphuncle is unknown. 



The inner volutions are three-fourths concealed, and the 

 outer one consequently appears to increase very rapidly in 

 dimensions, and is greatly larger than the others. 



This species is somewhat allied to Ammo?iites splendens, 

 pi. VII. fig. 10, but may at once be distinguished in being 

 destitute of tubercles on the inner margin of the volutions, 

 and is also devoid of the radiations, which ornament the 



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