304 THE LIAS AMMONITES. 



friend, Mr. John Leckenby, P.G.S., to whom the shell I have figured belonged ; it was 

 likewise the largest and finest example that had ever been found in Yorkshire. It is 

 now in the Woodwardian Museum, Cambridge. 



Locality and Stratigraphical Position. — This Ammonite was collected from the hard 

 shales of the zone of Amaltheus oxgnotus at Robin Hood's Bay, on the Yorkshire coast ; 

 the young form, figured and described by Prof. Buckman, F.G.S., as Am. Fowleri, was 

 collected in the shales of the Lower Lias, in the deep railway-cutting near the Lansdown 

 Station, Cheltenham. 



Arietites Collenotii, ctOrbigng. PI. VI, fig. I 1 ; PL XXII a, figs. 6—9 ; PL XXII b, 



figs. 1 — 3. 



Ammonites Collenotii, d'Orbigny. Pal. Franc., Terr. Jurass., p. 305, pi. xcv, figs. 6 



— 9, 1842. 



— denotatus, Simpson. Fossils of York. Lias, p. 76, 1855. 



— tenellus, Simpson. Ibid., p. 97, 1855. 



Arietites Collenotit, Tate and Blake. Yorkshire Lias, p. 290, 1876. 



Diagnosis. — Shell discoidal, compressed, volutions five to six ; whorls deep, less than 

 half the diameter of the shell, inner margin rounded, outer part depressed, sides costated ; 

 ribs straight for three- fourths of the whorl, then sharply bent towards the border ; 

 siphonal area small ; keel narrow and acute, with two broad lateral sulci ; sides convex, 

 bevelled towards the marginal area ; aperture narrow, deep, sagittate ; septa with three 

 lobes and three saddles. 



Dimensions. — Small specimen 'with shell (PL XXII a, figs. 6 — 9) : diameter 100 milli- 

 metres ; ditto of umbilicus 22 millimetres ; height of aperture 45 millimetres ; width of 

 ditto 23 millimetres ; height of keel 6 millimetres. 



Large specimen denuded of shell (PL XXII b, figs. 1 — 3) : diameter 140 millimetres; 

 height of the last whorl 65 millimetres; height of aperture 66 millimetres, width 33 

 millimetres ; width of the umbilicus 30 millimetres. 



Description. — This Ammonite has a compressed discoidal shell, provided with an 

 acute prominent keel. In early age the sides have close and sharp costae, which dis- 

 appear in the adult state. A specimen, PI. VI, fig. 1, 85 millimetres in diameter, has 

 forty straight acute ribs, which increase in thickness near the margin, and bend sharply 

 forward towards the aperture, entirely disappearing at the outer border of the sulcus. 

 PI. XXII a, fig. 6, is a specimen in which the shell is preserved ; it is covered with 

 minute striae, which run parallel with the ribs, and these delicate lines passing over the 

 keel give it a finely crenated appearance. The siphonal area is very narrow, from the 

 bevelling away of the sides towards the outer border ; the keel is acute and very promi- 

 nent (PL XXII a, figs. 6 — 8), and on each side has a broad, shallow sulcus, outwardly 

 1 Named Arietites denotatus on explanation of pi. vi. 



