AMMONITES. 49 



angustd, castas interrumpente, impresso : umbilico parviusculo : aperturd ovatd .- septum 

 lobis later alibus trifidis 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, 1^ 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. Old/iami, from which it is distinguished by its 

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

 Francaise,' M. D'Orbigny confounded this Ammonite with Sowerby's A. Lewesiensis 

 {A. leptopliyllus, Sharpe), and quoted it from the upper beds of the " Craie Chloritee," 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. Gollevil- 

 lensis they are trifid, with opposite and extremely regular branches. In A. leptopliyllus 

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



40. Ammonites curvatus, Mantell. Plate VII, figs. 8, 9 ; Plate XXIII, fig. ] . 



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



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



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



A. testa discoided, sub-compressd, tuberculatd, plerumque costatd .- anfractibus paucis 

 sub-anyulatis, lateribus depressis, tuberculatis ; dorso canaliculate-, utrinque bituberculato : 

 costis bicurvatis variantibus ; inter dmn paucis, latis, depressis; inter dum ad umbilicuin 

 numerosis, dorsum versus paucis, latisque ; scepe omnino obsoletis: tuberculis utrinque tri- 

 serialibus, ad umbilicum simplicibus, prope dorsum binis : umbilico parvo : aperturd rhom- 

 boided : septorum lobis later alibus tribus trifidis. 



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 



