HARPOCERAS RADIANS. 449 



whorls without the spiral angle and concave slope seen in the former. Harp, ovatum has 

 also a distinct prominent keel with a shallow groove on each side. 



Locality and Stratigraphical Position. — This species is collected from the jet-rock and 

 hard shales of the Upper Lias near Whitby and Runswick. Some very well-preserved 

 specimens are found in the ironstone nodules abounding in these strata. 



Harpoceras radians, Beineche. PI. LXIV, figs. 1—7 ; PI. LXXIV, figs. 1, 2 ; PI. 



LXXXI, figs. 4, 5, 6. 



Nautilus radians, Reinecke. Nautil. et Argonaut., figs. 39, 40, p. 71, 1818. 



Ammonites — Zieten. Versteiner. Wurtemh., p. 5, tab. iv, fig. 3, 1830, 



— — Buemer. Norddeutschen Oolithgebirg, p. 185, 1836. 



— — d'Orbigny. Pal. Franc., Terr. Jurass., t. i, p. 226, pi. 59, 1843. 



— — Quenstedt. Flbzgebirg. Wrirtembergs, p. 270, 1843. 



— — — Cephalopoden, p. 110, t. vii, fig. 4, 1849. 



— — Chapuis et Dewalque. Mem. Cour. Acad. Roy. Belgique Foss. 



Luxembourg, p. 72, pi. xi, fig. 1, 1854. 



— — Oppel. Juraformation, p. 247, 1856. 



— — Quenstedt. Der Jura, p. 281, table 40, figs. 13, 14, 1858. 



Diagnosis. — Shell discoidal, compressed, carinated with a wide umbilicus ; spire 

 formed of six volutions, depressed, slightly convex on the sides, of a regular elliptical 

 figure, and covered with from sixty to eighty round ribs, slightly prominent, always flexed, 

 and simple from the umbilicus upwards ; siphonal area narrow with a prominent keel, 

 well separated from the ribs, much elevated, and persistent in its form. 



Dimensions. — PI. LXIV, fig. 1. Transverse diameter 115 millimetres; width of 

 umbilicus 42 millimetres ; height of aperture 30 millimetres ; width of aperture 20 

 millimetres. 



Description. — The species of Ammonites composing the Radians group are closely 

 related to each other and require to be carefully studied at different periods of growth to 

 ascertain their affinities and differences with other congeneric forms, and also to 

 differentiate the species. The type of the Am. radians, Reinecke, fig. 39, is a well-defined 

 form ; the shell discoidal, compressed, and carinated, has a wide umbilicus, the spire is 

 composed of six volutions, which are nearly one half involute ; the whorls are gently 

 convex, and a transverse section of the outer whorl gives a fairly elliptical figure. There 

 are from sixty to eighty simple, blunt, biflexed radii on the last whorl, which extend singly 

 from the spiral suture to the area without bifurcation. The keel is prominent, con- 

 tinuous, and entire, it is present in very young as well as in old shells, and is well 

 separated from the flanks by a smooth or costate space, and in some varieties by a con- 

 siderable width of the area. This shell appears to undergo very little change of form 

 through life, a fact I have ascertained by breaking up several large Ammonites in order 



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