ARIETITES ROTIFORMIS. 279 



specimens, thus in Sowerby's and Zieten's shells there are twenty-seven, in von Hauer's 

 thirty-four, and in d'Orbigny's forty-five ; they are simple, strong, and arched, and 

 terminate near the margin in a large, round, obtuse tubercle. Sowerby says, " The ribs 

 rising in knobs just as they reach the front give the vi'horls a very square aspect ; the 

 ribs and the hollows between them are nearly equal. The siphonal area is wide, in 

 consequence of the extension given to this region by the size of the costal tubercles ; 

 upon the middle line of the back is a strong, thick, rounded keel, having on each side 

 thereof a deep sulcus, bounded by lateral elevations, so that the area is tricarinated with 

 two intervening sulci. The spire is composed of quadrate whorls, with prominent ribs, 

 and a series of nodules along the ciu:vature of the spire. The aperture is quadrate, 

 depressed, and sinuous. 



The septa of this species, according to Prof. A. d'Orbigny, are symmetrical, divided 

 on each side into four lobes, of which the two lower lateral are formed of two equal 

 parts. The siphonal lobe (PI. V, fig. 2), one third longer than the principal lateral 

 lobe, is ornamented on each side with a long denticulated branch. The siphonal saddle, one 

 third wider than the principal lateral lobe, is formed of three divided lobules, of unequal 

 size, of which the median is the largest. The principal lateral lobe terminates in two 

 points, and has on each side three or four others. The lateral saddle, narrower than the 

 principal lateral lobe, is formed of three unequal lobules, resembling those of the siphonal 

 lobe. The inferior lateral lobe, half the width of the principal lateral, terminates in two 

 points. The two auxiliary lobes are very small, and divide into one median and two 

 lateral points, A line, parting from the extremity of the siphonal lobe, passes at a con- 

 siderable distance frotn all the lobes externally visible, but touches the extreme spiral lobe, 

 which is bifurcated, and accompanied on each side by a very large columellar saddle. 



Affinities and Differences. — This species resembles in many characters A. Conybeari. 

 It is distinguished, however, from that form by having a wider siphonal area and fewer 

 ribs, each terminating in a large obtuse tubercle, the septa are likewise much more com- 

 plicated than those of A. Conybeari. It resembles A. bisulcatus, Brug., in possessing 

 simple ribs with marginal tubercles, and two deep sulci on each side of the median keel. 

 It is distinguished, however, from that form in having narrower whorls, a larger umbilicus, 

 and a very different lobe-line. Compare, for example, the sutures in PI. Ill, fig. 2, 

 with those in PI. IV, fig. 2. 



Locality and Stratigraphical Position. — This is a very rare Ammonite ; my specimen 

 was collected fi'om the Lower Lias, near Lyme Regis ; but its precise horizon I could not 

 ascertain. Sowerby's specimen was found in the Lias, near Yeovil. I have seen fragments 

 from a railway-cutting in the Lower Lias of Weston, near Bath, and from near Bristol, 

 Somerset. 



Foreign Localities. — Prof. d'Orbigny collected it from the Lower Lias, with Gryphcea 

 arcuata, at Pouilly, Cote-d'Or, where it is rare. The fine shell figured in PI. IX of 

 this work came from near that locality, and was presented to me by Messrs. 



