AMMONITES. 21 



Diameter of our only specimen 2| inches, breadth of the apertm-e fths of an inch, 

 height of the last whorl Ifths of an inch; but the specimen must have been much 

 larger, as it originally had, at least, one more whorl. 



Found by Mr. Wiest at Chardstock, in the Chalk with siliceous grains. 



This elegant shell has some characters which are so rare among the Ammonites, that 

 we only know one other species to which to compare it, A. complanatus, Sow. {A. largil- 

 liertianus, D'Orb.) ; it agrees with that species in general form, in the embracing 

 character of the whorls, covering the umbilicus, and in the cm've of the septa, which 

 advance towards the middle of the whorl, and fall back towards the ventral and dorsal 

 margins ; from this last character combined with the flexure of the ribs we conjecture that 

 the mouth had a sinus in the middle resembling that in the genera Nautilus and 

 Belleroplion. The lobes of the septa have been imperfectly seen ; they bear a strong 

 analogy to those of A. complanatus, without altogether agreeing with them. The de- 

 scription being taken from a single imperfect specimen is necessarily incomplete : this 

 specimen appears to be unique. 



3. Ammonites falcatus, Mantell. Plate VII, figs. 5 — 9. 



Ammonites falcatus, Mantell. Fossils of the South Downs, t. xxi, figs. 6 and 12. 



— — Sowerby. Min. Conch., t. 579, fig. 1. 



— — ITOrhigny. Paleont. rran9. Terr. Cret., t. xcix. 



— CDRVATUS, Mantell. Loc. cit., t. xxi, fig. 18. 



— — Sowerby. Min. Couch., t. 5/9, fig. 2. 



A. testa discoided, compressd, bicurvato-costatd, tuberculatd ; tuberculis, costisque 

 numero variantibus ; dorso canaliculato, utrinque tuberculato ; umbilico mediocri ; aperturd 

 ohlongd. 



Shell discoidal, and flattened, with numerous ribs extending from the umbilicus to the 

 edge of the back, in double curves, meeting in advancing angles on the middle of the 

 side, and variously tuberculated ; back with tubercles on each side, and a hollow canal 

 in the middle ; umbiHcus of middling size, bounded by steep sides ; whorls half con- 

 cealed ; aperture oblong ; septa with two lateral lobes, sUghtly subdivided. 



The above are the most constant characters in this very variable species, in which the 

 ribs are either very broad or minutely sub-divided, or replaced by tubercles. The best 

 marked varieties are those which were described by Mantell and Sowerby as distinct 

 species, although specimens may be found with all the characters of both varieties com- 

 bined, as fig. 5. A. falcatus originally included the forms without lateral tubercles, as our 

 fig. 7, and A. curvatus the more tuberculated forms, like fig. 6, which have two rows of 

 tubercles on each side of the back. We are indebted to M. D'Orbigny for uniting them. 

 The tubercles usually diminish in number and prominence, as the shell increases in 



