HARPOCERAS OPALINUM. 463 



the umbilicus, and widen as they pass singly over the sides, describing a double flexure 

 thereon and terminating in the lateral ridge which bounds the area. The umbilicus is 

 very narrow with vertical walls and shows all the inner whorls. 



The suture-line is complicated (PI. LXXXTI, fig. 10 a) ; the siphonal lobe is short and 

 narrow ; the siphonal saddle very large, with a long oblique accessory lobe which divides 

 the area into two spaces ; the external is the smallest and its walls are incised by six or 

 eight folioles ; the internal is nearly as large as the principal lateral, and here are twelve 

 folioles in the walls. The principal lateral lobe is very large and surrounded with ten 

 long denticles ; the lateral saddle is likewise large and the walls have many folioles ; the 

 external lateral lobe is much smaller than the principal, it has two side branched 

 denticles, and a terminal tuft ; the auxiliary lobes and saddles, four in number, diminish 

 in magnitude from without inwards, and present nothing of interest before reaching the 

 umbilicus. 



This species exhibits abroad many varieties in its young state ; some have the sides 

 flattened and the area angulated, others have sides and area gracefully curved, some have 

 the costse delicate, regular, and closely arranged, others unequal and deep, with distant 

 sulci between ; hence these various forms have been mistaken for other species. In the 

 adult condition, however, this Ammonite preserves a very uniform figure and ornamenta- 

 tion, and is a very characteristic fossil of the zone of Lyt. Jurense. 



Locality and Stratiyraphical Position. — The specimen figured (PI. LXXXII, figs. 

 9, 10) belonged to the late Dr. Lycett who collected it from the Lower Sands near 

 Nailsworth, it is now the property of the Museum, Geological Survey, Jermyn Street. 

 Pig. 11, PI. LXXXII, is copied from d'Orb., Pal. Franc. 'Terrains Jurassiques/ torn, i, 

 pi. 114, fig. 3. 



Harpoceras opalinum, Reinecke. PL LXXX, figs. 4, 5, 6, 7, 8. 



Nautilus opalinus, Reinecke. Nautil. and Argonaut., p. 55, figs. 1, 2, pi. i, 1818. 



— mjsandrus, — Ibid., p. 56, tab. 1, figs. 3, 4, 1818. 

 Ammonites primordialis, Schlotheim. Die Petrefactenkunde, No. 8, p. 65, 1820. 



— carinatus, Haan. Ammon. et Goniat., p. 139, 1825. 



— primordialis, Zieten. Versteiner Wurtemb., p. 5, tab. iv, fig. 4, 1830. 



— opalinus, Quenstedt. FlSzgebirge, p. 285, 1843. 



— — — Cephalopoden, p. 115, tab. /, fig. 10, 1849. 



— primordialis, d'Orbigny. Paleont. Franc., Terr. Jurassiques, p. 235, tab. 62, 



1842. 



— opalinus, Quenstedt. Der Jura, p. 307, tab. 42, fig. 8, 1858. 



— Lycett. Proc. Cotteswold Nat. Club, vol. iii, p. 5, 1865. 



Diagnosis. — Shell discoidal, compressed, and subcarinated ; volutions six, two thirds 

 involute ; sides slightly convex or flattened, inner margin concave, outer margin 



