Ammonites. 



MOLLUSCA. 



28 



Ammonites plicatilis. Sowerby, Min. Coneli. II. p. 149, 

 pi. lOG. Fltnniiig, Biit. An. p. 24± De la Beclie, Geo. 

 Maiuiel, p. 370. 



Discoidal, with six exposed volutions ; sides flat, crossed 

 by numerous, equal, straight ribs, wiiicii become bifurcate 

 or trifurcate towards the rounded ambit, wliicli is smooth in 

 the centre ; aperture quadrangular, witii rounded angles, a 

 little longer than wide ; septa acutely sinuatcd. Thieiiness 

 of the shell about one- fourth its diameter. 



So straight, close, and uniform are the ribs of this species, 

 that they have more the appearance of art tlian of nature; in 

 some cases, they are simple ; in others, bifid or trifid. 



Found in the Sandy Limestone of Dry Sandford and 

 Marsham, north west of Abingdon ; in the Kelloways Rock, 

 Coral and Oolite, Yorkshire ; the Coral Rag, middle and 

 soutii of England ; and in the same kinds of strata in various 

 places on the Continent. 



124. A. Jamesoni. — Jameson's Ammonite, pi. XVI. fig. 10. 

 Ammotiites Jamesoni. Sowerby Min. Conch. VI. p. 105. 



pi. 555, fig. 1. Phillips, Geology of Yorkshire, I. p. 163. 

 De la Beche, Geo. Manuel, p. 372. 



Discoidal ; with five or six volutions, the inner ones ex- 

 posed ; crossed by large, simple, obtuse, slightly bent ribs, 

 which curve forward as they pass over the rounded ambit, 

 meeting those on the opposite side, and are nearly equal to 

 the intermediate spaces ; aperture oblong-ovate, its length 

 being equal to twice its breadth. 



Discovered in the Lias, Island of Mull, by R. I. Murchison, 

 Esq. and named by him in honour of Professor Jameson of 

 Edinburgh ; it has also been found in the Lias of Yorkshire, 

 at Robin Hood's Bay. 



125. A. JoHNSTONi. — Johnston's Ammonite, pi. XVII. 

 fig. 1. 



Ammonites Johnstoni. Sowerby, Min. Conch. V. p. 70. 

 pi. 449, fig. 1. Fleming, Brit. An. p. 247. De la Beche, 

 Geo. Manuel, p. 375. 



Discoidal, with from six to nine greatly compressed, 

 one-third inserted volutions ; crossed by a central series 

 of numerous, short, straight, elevated ribs, or perhaps more 

 properlj', elongated tubercles ; inside of the volutions and 

 ambit plain ; central volutions destitute of ribs, and of nearly 

 equal thickness. 



When its outer coating is removed, this species exhibits 

 the most brilliant play of iridescent colouration. 



Discovered in the Lias Clay at Watchet, Somersetshire, 

 by Mr Johnstone, and named by Sowerby in honour of him ; 

 and has since been met with in the Lias at Bath. 



126. A ROTiFORMis. — Tlie Wheel-Shaped Ammonite, pi. 

 XVIL fig. 2. 



Ammonites rotiformis. Sowerby, Min. Condi. V. p. 76, 

 pi. 453. Fleming, Brit. An. p. 247. De la Beche, Geo. 

 Manuel, p. 375. 



Depressed, carinated ; sides somewhat concave, with from 

 six to eight entirely exposed volutions ; crossed by numerous, 

 distant, strong, slightly bent [elevated ribs, each provided 

 with an oblong, obtuse tubercle at its outer extremity ; ambit 

 flat ; keel slightly sunk, and with a furrow on both sides ; 

 aperture almost square, a little longer than wide, and only 

 one-sixth the diameter of the shell. Greatest diameter seven 

 inches. 



Found at Yeovil in the Lias, and also in the Lias at 

 Bath. 



127. A.BiFRONS — The Two-Fronted Ammonite, pi. XVII. 

 fig. 3. 



Ammonites bifrons. Piiilliiis, Geology of Yorksliire, I. p. 

 141, pi. 6, fig. 18. De la Beche, Geo. Manuel, p. 371. 



Discoidal, subumbilicate ; with three or four moderately 

 convex, rapidly increasing volutions, the inner ones one-third 

 concealed ; a series of remote, rounded, prominent, and nearly 

 straight ribs, emanate from tiie inner margin of the volutions; 

 and after reaching two-thirds across the sides, terminate, and 

 are met by numerous smaller ribs, which pass over the 

 rounded ambit, producing a erenulated profile. 



Found in the Kelloways Rock at Hackness, in Yorkshire. 



128. A. LONGispiNus Tlie Long-Spined Ammonite, pi. 



XVIL fig. 4. 



Ammonites longisjiinus. Sowerby, Min. Conch. V. p. 164, 

 pi. 501, fig. 2. Fleming, Brit. An. p. 247- 



Discoidal, tliick ; with three or four plain, half inserted 

 volutions ; sides furnished with two concentric series of spini- 

 form tubercles ; ambit thick, smooth, and gently rounded ; 

 aperture orbicular, deeply indented by the contiguous volu- 

 tion ; its greatest diameter somewhat more than three-fifths 

 of the diameter of the disk ; external surface pearlaceous. 



Found near Weymouth, Dorsetshire. 



129. A. CONTRACTUS. — The Contracted Ammonite, pi. 

 XVIL fig. 5. 



Ammonites contractus Sowerby, Min. Conch. V. p. 162, 



pi. 500, fig. 2. De la Beche, Geo. Manuel, p. 373. 



Suliglobose, deeply umbilicate ; inner volutions nearly 

 concealed ; sides contracted, appearing as if drawn into the 

 umbilicus, around whicli are a series of large rather acute 

 tubercles, from which emanate numerous smooth rounded 

 ribs, these branch into sets of three or four, and pass over 

 the greatly rounded back or ambit, and join those on the 

 opposite side ; aperture oblong, arcuated. 



Found in the Inferior Oolite at Dundry ; and in the same 

 strata, Normandy. 



130. A. Turner: Turner's Ammonite, pi. XVIL fig. 6. 



Ammonites Turneri. Sowerby, Min. Conch. V. p. 75, pl. 



452. Fleming, Brit. An. p. 244. De la Beche, Geo. Manuel, 

 p. 372. Phillips, Geology of Yorkshire, L p. 164, pl. 14, 

 fig. 14? 



Depressed, carinated ; with five volutions, the inner ones 

 almost entirely exposed ; sides flattened, with numerous equal 

 ribs, which continue almost straight until reaching nearly the 

 back, where they are suddenly curved forward ; carina but 

 moderately elevated, with a furrow on each side ; aperture 

 oblong, quadrangular, its lengtli being about one-third the 

 diameter of the disk. 



This shell is not unlike Ammonites Brookie, pl. XI. fig. 13, 

 but is distinguished by its more exposed volutions, in the 

 somewhat square aperture, and in tiie different curvature of 

 the ribs. 



Found in the Lias at Watchet, Wymondham Abbey, and 

 in tlie lower Shale of the Lias formation, Robin Hood's Bay, 

 Yorkshire. 



131. A. L.^viuscuLUS. — The Smoothish Ammonite, pi. 

 XVIL fig. 7. 



Ammonites Imviusculus. 

 pl. 451, fig. 1, 2. Fleming 

 Geo. Manuel, p. 373. 



Discoidal, carinated, umbilicate ; with four or five rapidly 

 increasing volutions, the inner ones being partly exposed 



Sowerby, Min. Conch. V. p. 73, 

 , Brit. An. p. 244. De la Beche, 



