Ammonites. 



MOLLUSCA. 



Genus IV.— AMMONITES.— ZamarcA. 



Shell cliscoifl, multilocular ; volutions contiguous, all 

 visible; inner partitions articulated by sinuous sutures; 

 septa transverse, lobed at the circumference and imper- 

 forated at the disc, but perforated by a single tube situate 

 near the margin. 



In the extensive genus Ammonites the situation of the 

 siphuncle is always upon the ambit or dorsal margin of 

 the transverse plates, as shewn in the perpendicular 

 section of Ammonites obtusus, pi. IV. fig. 8. It is repre- 

 sented in black, and marked by the letters c, d, e,f, g, h. 

 It is conducted through the plates by a ring, projecting 

 outwards, and may be traced passing through the whole 

 transverse plates of the above figure. The body of the 

 animal has occupied that portion of the shell from a to h. 



The Ammonites occur in all formations from the 

 transition strata, and disappear with the termination of 

 the Chalk. 



1. A. Stellaris — The Star Ammonite, pi. IV. fig. 2. 

 Sowerby, Min. Conch. I. p. 211, pi. 93. 



Involute, with four somewhat depressed volutions; ob- 

 tusely cariuated, on each side of which is a rounded furrow ; 

 inner volutions about two-thirds visible, with their sides flat- 

 tened, producing a pentagonal or stellated appearance ; with 

 numerous straight, moderately raised radiating ribs ; the 

 whole surface of the shell covered with obscure, rather remote 

 decussating strias ; sinuous margin of each septum crossed by 

 two costse ; the septa situated at each fourth rib ; siphuncle 

 placed in the keel ; aperture quadrangular, rather longer than 

 wide, its length being two-fifths tlie diameter of the shell. 

 Greatest diameter four and a half inches. 



This species is common at Lyme Regis, Dorsetshire. 



2. A. Lewesiensis. — The Lewes Ammonite, pi. IV. fig. 3. 

 Mantel], Geology of Sussex, p. 199, pi. 22, fig. 2. 

 Depressed ; three or four wide, flattened volutions, almost 



entirely concealed, and with four or five obscure, radiating 

 ridges ; septa sinuous, vei'y numerous, and the surface usually 

 covered with thin, foliaceous impressions; external volution 

 equal to four-sevenths of the diameter of the sIilU ; umbilicus 

 minute ; carene very narrow, rounded ; aperture sagittate. 

 Largest diameter usually about fourteen inches ; but speci- 

 mens have been found eighteen inches in diameter. Width 

 of the outer volution commonly nine inches ; greatest thick- 

 ness five inches, and at tiie external edge one and a half inch. 



The greatest thickness of this shell is at the inner margin, 

 from whence the volutions gradually taper to the keel. The 

 outer volution increases abruptly, and is nearly equal to half 

 the diameter of tiie shell. Mantell says, " This ammonite 

 may be readily distinguished in a suit of specimens, altliough 

 its characters are rather of a negative description. In its 

 general form it resembles Ammonites cowplanatus, ( Gray 

 Marie Fossils, No. 34 ;) but the umbilicus is larger, the 

 cavene less acute, and the surface exhibits no traces of striaj 

 or plicas. In the larger specimens the volutions appear to be 

 wholly inserted ; but probably, in more perfect examples, 

 their inner margin is exposed." 



Found in the Lower clialk near Lewes. 



3. A. BiNUS. — The Coupled Ammonite, pi. VII. fig. 11. 

 Sowerby, Min. Conch. I. p. 208, pi. 92, fig. 3. 



Involute, depressed ; volutions four ; the inner ones about 

 two-thirds exposed ; ribs radiating in pairs, emanating from 

 round tubercles, which are situate near tiie inner margin of 

 each volution, turgid, and then bent up towards the front, 

 where they become obsolete ; keel small, entire ; aperture 

 oblong, rectangular, one-third the diameter of the shell, and 

 a little more than one-sixth wide, with somewhat rounded 

 angles. In some specimens a single rib occurs between the 

 pairs. 



Found at Bramerton, Norfolk. 



4. A. STRiATUS. — The Striated Ammonite, pi. IV. fig. 6. 

 Sowerby, Min. Conch. I. p. 115, pi. 53, fig. 1. Goniatites 



striatus, Phillip's Geology of Yorkshire, II. p. 233, pi. 19, 

 figs. 1 — 3. 



Discoidal, very gibbose, its thickness being more than half 

 its diameter ; inner volutions entirely concealed ; outer sur- 

 face obscurely undulated transversely, and covered with nu- 

 merous fine, very regular, close, concentric striae, the undula- 

 tions traverse the surface in very regular semicircular 

 incurvations with the acute terminations meeting in points 

 upwards, and gradually passing into straighter lines on the 

 sides ; aperture semicircular, with nearly parallel margins ; 

 septa zigzag, rather remote, with four large, somewhat angular 

 folds ; siphuncle placed at the external margin of the septum, 

 where it is provided with a slight notch ; shell very thin. The 

 zigzag divisions are well marked, without passing into foliated 

 sutures as is usual in the genus Ammonites. Dorsal lobe bifid ; 

 dorsal sinus and first lateral lobe acute, and twice the length 

 of the dorsal lobe ; second lateral lobe obtusely rounded, 

 shorter than the first ; marginal sinus angular ; siphuncle not 

 continuous, but passing rectally from the septal plate for a 

 short distance. 



'1 he ribs and striae of the external shell are strengthened by 

 the repeated intersections of the subjacent edges of the trans- 

 verse plates. 



Found in Pools-hole in the Peak of Derbyshire, and in the 

 transition slate of Filiagh, near South-molton, Devonshire ; 

 BoUand, Flasby ; and also in Coal shale of Lough Allen in 

 Connaught, Ireland. 



5. A. sPH/EKicus — The Spherical Ammonite, pi. IV. fig. 7. 

 Martin, Petrefactions of Derbyshire, pi. 7, figs. 3, 4, and 5. 



Sowerby, Min. Conch. I. p. 116, pi. 53, fig. 2. Goniatites 

 sphcericus, Phillip's Geology of Yorkshire, II. p. 234, pi. 19, 

 figs. 4, 3, 6. 



Orbicular, diameter and thickness nearly equal ; inner 

 volutions entirely concealed ; outer surface with very fine 

 spiral striffi ; internal ridges variable ; septa with four broad, 

 angular folds, as in the preceding species ; but the lateral lobe 

 is less acute, or even rounded ; aperture a little contracted. 



Found in the Limestone of Derbyshire, and at Bolland ; 

 Isle of Man ; and in the county of Kildare, Ireland. 



This grows to double the size of ^. striatus. 



6. A. Mantelli. — Mantell's Ammonite, pi. IV. fig. 4, 9. 

 Sowerby, Min. Conch. I. p. 119, pi. 55. Mantell, Geology 



of Sussex, p. 113, pi. 21, fig. 9, and pi. 22, fig. 1. 



Discoidal, depressed, subumbilicate ; volutions three or 

 four, subrotund, about two-thirds concealed, margin trigonal ; 

 with numerous transverse tuberculate ribs, which alternately 

 reach entirely round the volutions, the shorter ones extending 

 about two-thirds across tlie volutions ; with from two to eight 

 rows of tubercles ; ambit flattish, provided with two rows of 

 marginal tubercles ; external edges of the septa with five 



B 



