280 THE LTAS AMMONITES. 



CoUenot and Breon, of Semur, to help me in my comparative work ; and I beg to return 

 these kind friends my best thanks. Professor Quenstedt says it is most abundant in 

 Germany, and is collected at Vaihingen, Mohringen, Graiind. Professor von Hauer's 

 figured specimen vras collected in the Yellow Kossen Beds at Enzesfeld, near Vienna. 

 Professor Studer found it in the Lower Lias of the Bernese Alps, and Lardy at Coulata 

 and Fondement, near Bex, in the valley of the Rhone. 



Arietites Brooei, Siowerhy. Plate VI, figs. 4, 5. 



Ammonites Beooki, Sowerhy. Min. Conch., vol. ii, p. 203, tab. 190, 1818. 



— — Zieten. Verstein. Wurttemb., p. 36, tab. xxvii, fig. 2, 1830. 



— Brookii, von Bueh. tTber ammoniten, p. 10, 1832. 



— — Quenstedt. Flozgebirge Wurttemb., p. 132, 1843. 



— Brooki, Simpson. Monogr. of the Ammonites, p. 53, 1843. 



— Brookii, Quenstedt. Cephalopoden, p. 80, 1846. 



— Brooki, Oppel. Juraformation, p. 83, 1856. 



— — Simpson. Fossils of the Yorkshire Lias, p. 97, 1855. 

 Arietites — Tate ^ Blake. Yorkshire Lias, p. 284, 1876. 



Diagnosis. — Shell depressed, strongly carinated, with a deep narrow sulcus on each 

 side of the keel ; whorls costated, with thirty-six strong, simple, arched, ribs, which curve 

 round the inner margin of the whorl, and terminate at the sulcus ; whorls half involute, 

 inner volutions w^ell exposed ; surface of the shell marked with fine flexed lines of growth ; 

 aperture oblong. ' 



Dimensions. — Transverse diameter of fig. 4, 140 millimetres ; height of the outer whorl 

 at the aperture 55 miUim. ; width 40 millim. ; amount of involution one half the whorl. 



Description. — This handsome, well-marked species is recognised by its compressed 

 shell, ornamented with strong, simple, arched ribs, that are large and prominent, and 

 equal in width to the valleys between them ; the ribs curve round the inner margin of 

 the whorl, arch regularly forward, and terminate at the areal angle of the outer margin 

 of the sulcus ; the siphonal area is sharp and angular ; the keel round and prominent, 

 with a deep furrow on each side, forming a well-marked boundary between the keel and 

 sides. The surface of the shell is marked with fine flexed lines of growth, corresponding in 

 flexure with those of the costge ; the siphuncle is lodged in the strong round keel. 



Affinities and Differences. — This fine, boldly marked shell so much resembles A. Turneri, 

 Sow., that it is difficult to discover the points of difference between them, whilst their 

 affinities are numerous. In A. Turneri the shell is thicker and more quadrate, the 

 siphonal area is broader, the ribs are nearly straight to the upper third of the whorl, 

 where they bend abruptly forward towards the aperture, and form a kind of thickened 

 eminence at the angle of flexure. In A. BrooM the upper third of the whorl slopes ofl", 

 and the ribs, which are not so numerous, arch more regularly forward, forming the 

 angle of flexure found in A. Turneri. This point of dift'erence may be only one of sex, 



