22 



MOLLUSCA. 



Ammonites. 



In the young shell, the septa are much less sinuated than 

 in the adult, which is finely exemplified by a specimen in the 

 possession of the Marchioness of Bath. 



Found in the Lias at Whitby, Yorkshire. 



Professor Buckland makes the following observations on 

 this interesting and curious species ; referring to its foliage, 

 he says, " Its laws of dentation are the same as in other Am- 

 monites, but the ascending secondary saddles, which in all 

 Ammonites are round, are in this species larger than ordinary, 

 and catch attention more than the descending points of the 

 lobes." 



The figures of the edge of one transverse plate are repeated 

 in each successive plate. The animal, as it enlarged its shell, 

 thus leaving behind it a new chamber, more capacious than 

 the last, so that the edges of the plates never interfere or 

 become entangled. 



Although the pattern on this Ammonite is apparently so 

 complicated, the number of transverse plates is but sixteen 

 in one revolution of the shell ; in this, as in almost all other 

 cases, the extreme beauty and elegance of the foliations result 

 from the repetition, at regular intervals, of one symmetrical 

 system of forms, namely, those presented by the external 

 margin of a single transverse plate. No trace of these folia- 

 tions is visible on the outer surface of the external shell, as 

 will be seen by our representation, pi. XVI. fig. 3. 



The elevations and depressions on the ambit of this species, 

 strongly illustrate the theory of Von Buch, respecting the 

 use of the lobes and saddles, formed by the undulations of 

 its outer margin.* 



118. A. FissicosTATUs. — The Cleft-Ribbed Ammonite, 

 pi. XVI. fig. 4. 



Ammonites Jissicostatus. Phillips, Geology of Yorkshire, 

 I. p. 123, pi. 2, fig. 4f). De la Beche, Geo. Manuel, p. 294. 



Discoidal, with five or six two-thirds exposed, somewhat 

 depressed volutions, crossed by numerous strong, slightly 

 undulated ribs, which become furcate about the centre, and 

 passing over the rounded ambit, meet and join with the cleft 

 ribs of the opposite side ; aperture subovate, its width being 

 about five-sixths of its length. 



Found in the Speeton Claj', Yorkshire. 



119. A. GEMMATUS The Gemmed Ammonite, pi. XVI. 



fig. 5. 



Ammonites gemmatus. Phillips, Geology of Yorkshire, I. 

 p. 141, pi. 6, fig. 17. De la Beche, Geo. Manuel, p. 371. 



Discoidal, with four or five ventricose volutions, the inner 

 ones half exposed, and much thinner as they approach the 

 centre ; inner margin of tiie volutions slightly flattened, from 

 which emanate numerous, somewhat distant, strong, elevated, 

 nearly straight ribs, which, on reaching the centre of the 

 sides, are united to large, rounded, obtuse tubercles ; from 

 which tlie ribs are continued diagonally, and terminate in 

 large, sliglitl3' conical tubercles, on both sides of the broad and 

 flattened ambit ; between each of the exterior half of these 

 ribs and the back, are two or three smaller costse, which also 

 terminate on the ambit. Greatest diameter six inches. 



Discovered in tiie Kelloways Rock, Yorkshire. 

 ~ 120. A. subljEvis. — Tlie Half Smooth Ammonite, pi. XVI. 

 fig. 6. 



Ammonites sublffvis. Sowerby, Min. Conch. I. p. 117, pi- 

 54. Phillips, Geology of Yorkshire, I. p. 141, pi. 6, fig. 22. 



• See our Elements of Fossil Conchology, where this theory is fully 

 explained. 2 



De la Beche, Geo. Manuel, p. 370. Orbulites Icevis, Lamarck, 

 54. Oibulita modiolaris, Fleming, Brit. An. p. 248. NaiUilus, 

 &c. Lnidii Lithoph. Brit. Tab. 6, fig. 292. 



Orbicular, unibilicate ; inner volutions all enveloped in the 

 exterior one, and their crenated edges visible only within the 

 deep conical umbilicus, which is undulated internally, and 

 provided with a sharp angular edge ; aperture semicircular, 

 very wide, and truncated at the sides ; whole outer surface 

 smooth, and devoid of inequalities ; septa numerous, with fine 

 prominent undulations, which are divided into many lesser 

 and rounded foliations, their margins resembling pinnate 

 leaves ; siphunculus close to the outer margin of the ambit ; 

 diameter, five inches, umbilicus, an inch and a quarter in 

 width. 



This species varies much in its j'oung and adult state. 

 When young, it is considerably compressed, and its surface 

 provided with slight furcate undulations. In the infant state, 

 its width is only half its length, and exhibits, on its surface, 

 transverse ribs ; these become less acute, and assume a more 

 rounded and depressed form, as the shell progresses, and 

 consist of longer and shorter ones, some of which are 

 bifurcate. When the shell has reached three inches, these 

 become less conspicuous, and when it has grown to four or 

 five inches in diameter, they are entirely obsolete.* 



Found in the Coral, Oolite, and Kelloways Rock, York- 

 shire, and Middle and South of England ; the Fuller's Earth 

 of Bath ; also in the Oxford Clay of Begginger, Schafen- 

 hausen, according to Von Buch ; and De la Beche says it is 

 found in the same kind of strata, Normandy. 



121. A.FLExicosTATUs. — The Bent-Ribbed Ammonite, pi. 

 XVI. fig. 7. 



Ammonites flexicostatus, Phillips, Geology of Yorkshire, 

 I. p. 142, pi. G, fig. 20. De la Beche, Geo. Manuel, p. 371. 



Discoidal, compressed ; with four or five two-thirds ex- 

 posed volutions, slightly raised in the centre of the sides, and 

 slanting off to the margins, crossed by numerous, elevated, 

 bent ribs, which become furcate in the middle, and passing 

 over the somewhat thin and flattened ambit, meet with and 

 join the furcations on the opposite side ; the outer volutions 

 increase rather rapidly ; aperture oblong-ovate. 



Found in the Kellowaj's Rock at Hackness, Yorkshire. 



122. A. CRAssus.— The Thick Ammonite, pi. XVL fig. 8. 

 Anunonites crassus. Phillips, Geology of Yorkshire, I. p. 



163, pi. 12, fig. 15. De lu Beche, Geo. Manuel, p. 371. 

 Young and Bird, Geo. Yorkshire, p. 



Discoidal, with five thick half inserted volutions, somewhat 

 flattened on their sides, crossed by numerous, straight, pro- 

 minent, rounded ribs, which, on reaching the centre, are pro- 

 vided with a round, elevated tubercle, from which the ribs 

 split into two or three divergent smaller costse, and passing 

 over the thick rounded ambit, unite with those of the opposite 

 side ; aperture, subcordiform. 



This species has some affinity to Ammonites JBrodiai, pi. 

 X. fig. 13, but may at once be distinguished from it by its 

 inferior size, its greater proportional thickness, in having 

 fewer volutions, and the central ones being thicker in all stages 

 of growth, and in the external minor ribs being fewer and 

 less elevated. 



Found in the upper Shale, Lias formation, of Yorkshire. 



123. A. PLicATiLis The Plaited Ammonite, pi. XVI. 



fig. 9. 



• See our Elements of Fossil Conchology. 



