Ammonites. 



MOLLUSCA. 



11 



surface of the former species. It is, liowever, like tliat shell, 

 frequently iridescent on the surface. 



Found at Ringmer, by G. Mantcll, Esq. 



38. A. coRDATus The Cordate Ammonite, pi. VIII. 



fig. 4. 



Ammonites cordatus. Sowerby, Min. Conch. I. p. 51, 

 pi. 17, figs- 2 and 4. 



Involute, carinated ; volutions four or five, somewhat 

 compressed, inner ones half inserted ; sides ornamented with 

 undulating ribs, extending over the inner half of each volu- 

 tion ; the remaining half provided with about five divergent 

 undulations to every two ribs, all of which terminate in the 

 exterior crenated margin ; aperture cordiform, two-thirds of 

 the diameter of the shell in length. Diameter varj'ing 

 from one to two inches ; thickness about a third of its 

 diameter. 



Found in the Limestone of Shotover, Oxfordshire, and 

 also in Somersetshire. 



39. A. Browni Brown's Ammonite, pi. VIII. fig. 5. 



Ammonites Broivni. Sowerb}', Min. Conch. III. p. 114, 



pi. 163, figs. 4, 5. 



Discoidal, carinated ; five half inserted volutions, with 

 a zone of large distant tubercles placed towards the centre 

 of the volutions, but rather nearest the inner sides ; these 

 assume the form of ribs on the outer volutions ; from the 

 tubercles, the other half of the volutions are provided with 

 numerous equidistant, somewhat curving ribs, which extend 

 over the rounded ambit; aperture cordiform. 



Found at Dandr}', by G. W. Braikenridge, Esq. and 

 named in honour of Robert Brown, Esq. the celebrated 

 botanist. 



This species has much the appearance of Ammonites 

 Kcmigi, pi. IX. fig. 2. ; but the keel and tubercles upon the 

 inner volutions sufficiently distinguish it from that shell. 



40. A. ANNULATUS. — The Ringed Ammonite, pi. VIII. 

 fig. 6. 



Ammonites annulatus. Sowerby, Min. Conch. III. p. 41, 

 pi. 222. 



Discoidal, with from five to seven exposed volutions, 

 crossed by numerous, close, very prominent ribs, which are 

 frequently bifurcated as they pass over the rounded ambit ; 

 aperture subrotund. 



Found at Whitby, Yorkshire ; in the lower sand beds of 

 the inferior Oolite at Cropredy, near Bunbury, Oxfordshire, 

 and also near Ilminster. 



This species, at first appea.rance, has somewhat the aspect 

 of Ammonites communis, pi. VI. fig. 9. ; but its numerous 

 ribs sutficientlj' distinguish it, and, besides, it has more volu- 

 tions. The ribs are placed very near each other, and a deep 

 furrow is formed between them ; some being bifurcate as 

 they pass over the ambit. Sowerby says, " When the outer 

 surface of the shell, — which adheres strongly to the stone, — is 

 broken off, the ridges are much diminished ; and, instead of 

 convex surfaces, like wire wound about the shell, they are 

 flat, as if they were formed of square wire. The cast, when 

 all the shell is removed from it, is also marked by slightly 

 elevated radii." 



In some specimens, the sides of the volutions are somewhat 

 compressed ; in others, they are a little inflated ; in these 

 separate conditions, they look considerably difl'erent, but 

 may be at once recognized by the numerous strong annu- 

 lations. 



41. A. cuRVATus The Bending Ammonite, pi. VIII. 



fig. 7, and pi. X. fig. 12. 



Ammonites curvatus. IMantell, Geology of Sussex, p. 118, 

 pi. 21, fig. 8.; Sowerby, Min. Conch. VI. p. 154, pi. 179, 

 fig. 2. 



Discoidal, carinated, compressed, subuml)ilieate, with three 

 deeply inserted volutions, which are ornamented by trans- 

 verse, falciform, numerous ribs ; these are bifurcated at their 

 commencement, and terminate in broad, curved, tubercular 

 costoe ; keel with a longitudinal sulcus, situate between two 

 marginal series of tubercles ; ambit flat and narrow ; umbi- 

 licus large, aperture obtusely sagittate ; siphuncle situate in 

 the dorsal furrow. 



This species is nearly allied to the following, but appears 

 quite distinct. IVlantell says, " The curvatures are more 

 numerous in the Ammonites falcatus than the oblique radii ; but, 

 in the present species, the proportions are reversed, two or 

 three radii uniting to form one curved rib. The terminations 

 of the ribs in the latter are tubercular, and separated from 

 each other by a sulcus ; in the former, they are gently curved, 

 and appear as if folded or plaited over each other." 



The umbilicus is somewhat deeper than in Ammonites fal- 

 catus, and is provided with a row of oblique tubercular pro- 

 cesses, from each of which two or three ribs emanate, and 

 continue to the centre of the volutions, where thej' unite, to 

 form a broad curved rib, that terminates in an oblong-ovate 

 tubercle on the margins of the ambits. Another tubercle is 

 situate on the middle of the curved parts. The keel is 

 grooved, and has two belts of prominent, distinct opposite 

 tubercles formed by the terminations of the ribs. 



Discovered at Hamsey bj' Mr Mantell. 



42. A. FALCATUS. — The Hooked-ribbed Ammonite, 

 pi. VIII. fig. 8. 



Ammonites falcatus. Mantell, Geology of Sussex, p. 117, 

 pi. 21, figs. 6 and 12; Sowerby, Min. Conch. VI. p. 153, 

 pi. 579, fig. 1. 



Discoidal, carinated, greatly compressed, subumbilicate ; 

 witli three deeply inserted volutions, flat on both edges ; 

 sides furnished with numerous clo-^e, plicated, falciform ribs, 

 extending a little way down the sides of the umbilicus, which 

 is small, and with crenulated margins ; ambit flat, narrow, 

 and provided with a longitudinal sulcus ; margin plicated ; 

 aperture sagittate ; siphuncle placed in the furrow, which is 

 in the centre of the dorsum. 



This handsome species is nearly flat, its longest diameter 

 exceeding its greatest thickness almost four-fifths ; the sides 

 are slightly inflated in the centre, but are contracted at the 

 ambit into a narrow flattened carina, with a sulcus in its 

 centre, and witli the edges slightly plicated ; the ribs are 

 extremely slender at their origin in the umbilicus, but gra- 

 dually increase in breadth as they approach the centre of the 

 volutions, where they become suddenly curved, and sweep 

 elegantly towards tiie dorsal margin, where they terminate in 

 obtuse folds. 



Found at Middleham and Stonehaven, Sussex, in the Gray 

 Chalk Marie. 



43. A. Brocchii Brocchi's .4.ramouite, pi. Vlil. fig. 9. 



Ammonites Brocchii. Sowerby, Min. Conch. II. p. 233, 



pi. 202. 



Compressed, with three or four greatly rounded volutions; 

 the inner ones half concealed ; sides hollow ; ambit circular ; 

 a row of oblong-ovate ribs commence near the inner margin 



