175 



differently curved radii, distinguish this from A. Brookii, to which it 

 bears a strong resemblance." If a larger series of both forms could be 

 obtained, it is probable it would be found that they merged into one 

 species ; but, for the sake of convenience and easy reference, until 

 more facts are brought to light, I have thought it best to figure on the 

 same plate a good type form of each, from which it will be seen that the 

 affinities are many, and the differences few and immaterial. 



Locality and Stratigraphical Position. — Ammonites Broohii is found at 

 Lyme Regis in fine preservation, with the shell complete, in beds of 

 Lower Lias Limestone ; sometimes it is enclosed in crystalline masses of 

 the Sulphuret of Iron. It is on the whole a rare Ammonite. 



Ammonites Sauzeanus, d'Orbigny. (PI. 2, fig. 1, a, &, c, c?.) 



Ammonites Sauzeanus, d'Orbigny, Paleontologie Fran^aise Ter. Jur., 



PL 95, fig. 45, p. 304. 

 II II Wright, Monograph on the Asteroidea, Pal. Soc. 



vol. 1862, p. 22. 

 II halecis, Buchnian, Murchison's Geol. of Cheltenham, 



PI. 11, fig. 9, p. 104. 



Shell compressed, whorls quadrate, one third involute and ornamented 

 with twenty costse ; the ribs are slightly bent, the convexity backwards, 

 and they terminate in a tubercle at the dorsal angle ; back flat with an 

 elevated centi-al ridge. 



Dimensions. — The large specimen, transverse diameter 8 inches, height 

 of aperture 2^^ inches, width of aperture 2/^ inches. 



Description. — This remarkable Ammonite has an extremely quadrate 

 physiognomy, the whorls about one third involute, are square and 

 ornamented in the large specimen figured with twenty ribs, which are 

 simple, slightly bent vdth their convexity backwards, and terminate at 

 the angle of the back in small well-marked tubercles, PI. 2, fig. 1. The 

 back is large and square, an inconsiderable eminence rises in the middle line, 

 which, however, does not form a keel, nor are there any traces of latei-al 

 sulci as in the true keeled Aiietes. The spire, formed of square shaped 

 whorls, imparts a stair-like character to the open umbilicus. The 

 mouth is quadrate, the width slightly exceeding the height, the 

 septa are symmetrical, foliated on each side, and divided into four lobes 

 with narrow branches; the dorsal lobe (fig. 1, c.) long and narrow, 

 projects beyond the others, the outer side has three short simple, 

 two long branched, and one terminal branched digitation; on the inner 

 side of each bilateral segment of the lobe there are five or six simple 



