8 JURASSIC FAUNA. 



In having a small umbilicus, and a flattened ventral side, which, however, is not 

 quite so flat as in Nautilus giganteus, but is slightly rounded. 



Professor Waagen states that this form is closely allied to Nautilus calloviensis, 

 Oppel, but differs from it by a much larger umbilicus. Having the originals of both 

 of Professor Waagen's types before me, I think I should have taken another view, 

 because the original of Nautilus calloviensis is a fragment, only half the size of 

 Nautilus wandaensis, and it might perhaps be assumed that, if fully grown, would 

 possess an umbilicus of the same diameter as Nautilus wandaensis. 



Nautilus giganteus, d'Orbigny. Plates IV and V. 



1842. Natitilus giganteus, d'Orbigny, Pal^ontologie franf aise. Terrains jarsBsiqnes, Vol. I, p, 163, pL 36. 



1825. Nautilus giganteus, d'Orbigny, Annales des sciences naturelles, Vol. 5, pi. 6, fig. 2. 



1826. Nautilus hexagonus, Sowerby, Min. Conch., Vol. VI, p. 55, pi. 529. 



1875. Nautilus Jcumagunensis, Waagen, Pal^eontologia Indica, Jurassic Fauna of Eutob, Vol. I, p. 19, pi. Ill, 

 fig. la, b. 



Dimensions. I. 11. III. 



Diameter of tbe shell 223 222 155 



„ of the umbilicus >....... 47 40 30 



Height of the last whorl from the umbilical suture . . .110 121 91 



„ ), ,, from the preceding whorl . . . P 93 68 



Thickness of the last whorl 145 142 102 



Whorls thick, but comparatively low ; slightly compressed laterally ; ventral 

 side flatly concave and bounded by obtuse margins. Whorls thickest near the 

 umbilical margin ; aad sloping gradually towards the siphonal side, but dropping 

 steeply towards the umbilicus. The latter is very narrow and deep, the umbilical 

 margin rounded, and the umbilical wall almost perpendicular. 



The siphuncle is eccentric and nearer the dorsal side. 



The septa are simply curved but rise considerably at the ventral side, where 

 they are strongly cut out in the middle. 



Locality and stratigrapUcal position. — ^Mazdr Drik ; Po/g^p^emws-limestone. 



Bemarks. — If we consider with d'Orbigny the slightly excavated ventral side, 

 and the very slightly sinuous sutures as the characteristics of Nautilus giganteus^ 

 then it is quite clear that this form must be united with Nautilus hexagonus 

 Sowerby. On the other hand, I can see no points of difference between Nautilus, 

 Jcumagunensis, Waagen, and Nautilus hexagonus. This form has been founded by 

 Waagen on a single specimen, which apparently represents a young individual. 

 I think that it cannot be separated from Nautilus hexagonus. Sow., as the ridges on 

 either side of the siphonal side are sufficiently developed to indicate that type. Nor 

 do 1 think that any conclusion could be drawn from it as regards the size of the 

 ■umbilicus of a full-grown specimen, for, as I have aleady said. Professor Waagen's 

 specimen is apparently a young one. It certainly exhibits the same characters as 

 the enormous specimens from Ealuchistdn which undoubtedly show a fairly wide 

 umbilicus. Por this reason I have considered it identical mih. Nautilus hexagonus^ 

 Sow., which again is identical with d'Orbigny's Nautilus giganteus. 



