Ammonites. 



MOLLUSCA. 



Depressed, with usually six volutions ; the surface covered 

 with numerous, sometimes furcated well rounded ribs, and 

 intermediate shorter ones extending to half the breadth of 

 the volutions ; inner volutions exposed ; sides somewhat 

 straitened ; aperture oliovate ; septa numerous, with greatly 

 sinuated margins. Thickness about equal to one-fourth of 

 its diameter. 



This species is the largest of the genus. There is a spe- 

 cimen in the Museum of the Jardin des Plantes, Paris, four 

 feet in diameter. One was said to have been broken at 

 Chicksgrove quarry, near Hindon, Wiltshire, iu a compact 

 sandj' limestone, which was as large as the hinder-wheel of a 

 carriage. Specimens two feet in diameter are not uncommon. 

 It is found, besides the above locality, at Peerbeck Isle, 

 Dorsetshire ; Marleborough Downs, in the Chalk near Mar- 

 gate ; and at Fonthill. 



14. A. ELLiPTicus — The Oval Ammonite, pi. V. fig. 7- 

 Sowcrby, Min. Conch. I. p 209, pi. 92, fig. 4. 

 Depressed, with a sharp keel ; the interior volutions two- 



tliirds exposed ; ribs few, distant, broad, flat, agreeing in 

 number with the septa, and slightly curved, somewhat obso- 

 late near the margin ; aperture oblongly elliptical. 

 Found in the Mai'ley clay at Charmouth. 



15. A. coiiNuoiDES. — The Little-horn Ammonite, pi. V. 

 fig. 8. 



Involute, depressed, with a broad, flattened keel ; the 

 whole surface covered by rather prominent, gently bending, 

 distinct ribs, extending from the internal margin to the 

 carina, thickening outwards ; inner volutions considerably 

 exposed ; aperture subcordate. Diameter an inch and a 

 quarter ; tliickness three-eighths. 



Found at ^Vhitby. 



16. A. TRiPLicATUs. — The Three Pleated Ammonite, pi. 

 V. fig. 9. 



Ammonites triplicatus. — Sowerby, Min. Conch. III. pi. 292, 

 and 293, fig. 4. 



Discoid, with six exposed volutions, the two external ones 

 separated by a depression or flattened spiral groove ; the 

 whole external surface covered by strong, equidistant, regular, 

 slightlj' bent ribs, extending from the interior side to nearly 

 the external side, where they cease, the spaces between them 

 being greater than the thickness of the ribs ; aperture sub- 

 cordate. Diameter eight inches, and equal to four times its 

 thickness. 



Found near Malton, Yorkshire, and in the Suffolk Clay. 



17. A. BiPLEx. — The Two-Pleated Ammonite, pi. V. 

 fig. 10. 



Sowerby, Min. Conch. III. p. 167, pi. 293, fig. 1, 2. 



Discoid, with six exposed volutions, all separated by a 

 depression or flattened groove ; furnish(^d with large equi- 

 distant, regular elevated ribs, extending in a straight line 

 from the margin of the separating groove to two-thirds 

 across the volutions, where they are furcated, and pass 

 over the dorsal margin, which is rounded ; aperture oblong, 

 subcordate. Diameter eight inches ; thickness a fourth of 

 its diameter. 



Found in the Suffolk Clay, and also in the London Clay. 



18. A. Brongniarti Brongniarte's Ammonite, pi. VI. 



fig. 1. 



Sowerby, Min. Conch. IL p. 190, pi. A. fig. 2. 

 Gibbous ; thickness about two-thirds its diameter ; with a 

 minute umbilicus ; round within, but externally oblong, pro- 



duced by tiie line of last volution, being straight for a little 

 distance, from whence it makes a sudden turn towards the 

 aperture ; inner volutions concealed ; whole surface covered 

 with close, undulating, very regular, rather depressed, fur- 

 cated radii ; aperture placed transversely, provided with a 

 thick inflected lip. 



Found at Yeovil and in the Marley Limestone, Normandy. 



19. A. Calloviensis The Kelloways Ammonite, pi. \T. 



fig. 2. 



Sowerby, Min. Conch. II. p. 3, pi. 104, fig. 1. 



Involute, subumbilicate, with five volutions, three-fourths 

 concealed ; front, or ambit, depressed ; with very numerous, 

 small, bent, radiating ribs, arranged in sets, with a stronger 

 one reaching across the volution, and from two to five 

 shorter ones, alternating with a longer rib over the whole 

 surfiice ; these are somewhat obscure in the external volutions 

 of adult shells, in which the aperture is deltoidal, with 

 truncated angles, but obicular in young specimens ; siphuncle 

 placed near the upper edge. Greatest diameter three 

 inches. 



Found in the Shell-Limestone at Kelloway's Bridge. 



The form of the volutions in this species is much influenced 

 by age. When young, they are somewhat rounded, with 

 numerous sharp ribs arranged in sets ; a series of produced 

 ones, between every two of which are placed from two to 

 five shorter and more depressed costre, reaching about two- 

 thirds across the volutions ; the whole ribs passing over the 

 flattened ambit. The outer volutions of adult specimens are 

 triangular, the two inner angles being truncated, producing 

 an umbilicated aspect ; the surface with large undulations, 

 wrinkled near the ambit, and provided with numerous irre- 

 gular stritE in place of ribs ; they differ also in the inner 

 surface of the outer volutions being destitute of striae, and in 

 losing the ribs sooner. The shell is thick and is frequently 

 well preserved. 



20. A. Gervilii De Gerville's Ammonite, pi. VI. fig. .3. 



Sowerby, Min. Conch. II. p. 189, pi- A, fig. 3. 



Gibbous, largely umbilicate, exposing the ribbed margins 



of the volutions ; thickness somewhat more than half the 

 diameter ; with sharp, numerous, close, very regular, bent, 

 furcated ribs, continuing so to near the completion of the 

 last volutions, when they are supplanted by two or three 

 irregular undulations ; inner volutions but slightly exposed ; 

 aperture transversely oblong, and excavated ; lip sharp on 

 the edge, and arched. 



Found in Marley Limestone. 



21. A. oBTUsus — The Obtuse Ammonite, pi. VI. fig. 4. 

 and pi. IV. fig. 8. 



Sowerby, Min. .Conch. II. p. 151, pi. 167; Buckland's 

 Bridgewater Treatise, I. p. 347, and II. p. 58, pi. 35, 36. 



Discoidal, with an obtusely rounded, considerably elevated 

 keel, with a slight furrow on each side ; furnished with six 

 volutions, the inner ones wholly exposed, covered with large, 

 curved, remote, slightly elevated, strong ribs, equal in 

 number to the septa ; each crossing the inner lobes of a 

 septum ; somewhat sharp in the middle ; aperture oblong, 

 longer than wide, about equal to one-third the diameter of 

 the shell. Largest diameter five inches and a half. 



Found iu the Lias at Lyme Regis, Dorsetshire. 



Sowerby mentions a specimen from which he made his 

 drawing, sent to him by Miss Philpot of Linlej', " which, 

 from the high polish and rich colour of the crystallized 



