AMMONITES. 49 



angustd, castas inierrumjpente^ impresso : umbilico parviusculo : aperturd ovatd : septum 

 lobis lateralihus trijidis ramosissimis. 



Shell discoidal, whorls smooth and flattened on the sides, and ribbed on each side of 

 the rounded back ; ribs short, numerous, and equal, commencing beyond the middle of 

 the side of the whorl, and suddenly interrupted at the middle of the back by a narrow, 

 impressed line : umbilicus rather small, allowing about one third of the inner whorls to be 

 seen : aperture oval : septa very numerous, with minutely subdivided lobes ; lateral lobes 

 trifid, with regularly opposite branches. 



Diameter, IJ inch; height of the last whorl, 2 inches; width of the aperture, 

 \\ inch. 



Rare in the hard Chalk of the county of Londonderry, which is supposed to be of the 

 age of the Upper Chalk of England. 



This species is closely related to A. Oldhami, from which it is distinguished by its 

 short ribs, nearly confined to the back, and its smooth sides. In the * Paleontologie 

 Fran^aise,' M. D'Orbigny confounded this Ammonite with Sowerby's A. Lewesiensis 

 {A. lepfophyllus, Sharpe), and quoted it from the upper beds of the " Craie Chloritce," or 

 on the level of our Grey Chalk. In his ' Prodrome ' he separates it from that species, 

 and considers the beds in which it is found as part of the Upper Chalk. 



It is difficult to understand how two such different species could ever have been con- 

 founded, as the only character common to both is the impressed dorsal line. The lobes 

 of the septa of both are much subdivided, with spiked terminal digits, but in A. GoUevil- 

 lensis they are trifid, with opposite and extremely regular branches. In A. lep)top)liyllus 

 they bifurcate several times with very unequal alternating branches, and great irregularity. 



40. Ammonites ctjrvatus, Mayitell. Plate VII, figs. 8, 9 ; Plate XXIII, fig. ] . 



Ammonites curvatus, Mantell. Fossils of the South Downs, p. 118, t. 21, fig. 18. 



— — Sowerbij. Miu. Conch., t. 5/9, fig. 2. 



— FALCATUS? Pictet and Roux. Gres Verts des Env. de Geneve, t. 5, fig. .5. 



A. testd discoided, sub-compressd, tuherculatd, jjlerimque costatd : anfractihus paucis 

 suh-angulatis, lateribus depressis, tuberculatis ; dorso caiialiculato, utrinque bituberculato .• 

 costis bicurvatis variantibus ; interdum paucis, latis, depressis ; interdum ad umbilicum 

 numerosis, dorsum versus paucis, latisque ; scepte om7iino obsoletis: tuberculis utrinque tri- 

 serialibus, ad umbilicum simplicibics, pr ope dorsum binis : 2cmbilico parvo : aperturd rhom- 

 hoided : septorum lobis lateralibus tribus trijidis. 



Shell discoidal, flattened on the sides and back, with very few nearly rhomboidal 

 whorls ; strongly tuberculated, and frequently ribbed : back with two rows of tubercles on 



