174 



at Fretheme Cliffs, on the banks of the Severn, and found fragments of it 

 in the Lower Lias Limestones of the counties of Somerset, Gloucester, 

 Worcester, and Warwick. The finest specimens I have found were on the 

 coast near Lyme Regis, and in the Lower Lias, at Keynsham, near Bath; 

 the sjiecimen figured was obtained from a railway cutting in the Lower 

 Lias, near Brigg, in Lincolnshire. 



Ammonites Brookii, Sowerhj. (PI. 2, fig. 5.) 

 Ammonites Brookii, Sowerhj, Min. Conch., vol 2, tab. 190, p. 203, 1818, 

 ., 1. Zieten, Petrif. Wurtemb.,tab. 27, fig. 2, p. 36, 1830. 



II II Ojypel, Juraformation, p. 83, 1856. 



It II Simpson, Yorkshire Lias, p. 97. 1855. 



Shell depressed, strongly carinated, -with a deep nan-ow srdcus on each 

 side of the keel ; whorls costated, with 36 strong, simple, arched, ribs, 

 which curve round the inner margin of the whorl, and terminate at the 

 sulcus; whorls half involute, inner volutions well exposed; sui*face 

 of tlie shell marked with fine flexed lines of growth ; aperture oblong. 



Dime7mons. — Tranveree diameter 5 J inches; height of the outer whorl 

 at the apei-ture 2-^ inches; mdth 1 J inch; amount of involution one half 

 the whorl. 



Description. — This handsome, well-marked species is recognized 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 

 forwards, and terminate at the dorsal angle of the outer margin of the 

 sulcus; the back is sharp and angular; the keel round and i^rominent, 

 with a deep fuiTow on each side, forming a well-marked boundary 

 between the keel, back, and sides. The surface of the shell is marked 

 with fine flexed lines of gi'owth, corresponding in flexure with those 

 of the costse ; the siphuncle is lodged in the strong round keel. 



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

 resembles A . Ttirneri, Sow., that it is difficult to discover the points of 

 difference between them, whilst the affinities are numerous. In A. Turneri 

 the shell is thicker and more quadrate, the back 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. Broohii the upper third of the whorl slopes 

 off, and the costae, which are not so numerous, arch more regularly 

 forward, forming the angle of flexure found in A. Turneri. This point 

 of difference may be only one of sex, for Sowerby says in his description 

 of A. Turneri, — "The more exposed whorls, squarish apci-ture, and 



