87 



LIX. Nerita. A semi-globose univalve, depressed beneath, and 

 having no umbilicus : the opening entire and semicircular ; colu- 

 mella nearly transverse and flat, with an acute, and generally dentated 

 edge. 



The shells of this genus differ from those of Natica, in never being at 

 all umbilicated. 



Nerita conoidea. Lam. Nerita perversa, Gmelin, is, as well from its 

 form, as from the extraordinary magnitude which it sometimes possesses, 

 a very remarkable fossil. It is conical, with a very broad base; the 

 apex of the spire is inclined, and the columella is furnished with eight 

 teeth. 



Chemnitz, and other naturalists, have thought that this was a reversed 

 shell ; but Lamarck has shown that its turns are in the ordinary direc- 

 tion, from the left to the right. It acquires, however, a peculiar ap- 

 pearance, from the top of the spire being inclined to one side, as if the 

 axis of the spire had been broken or bent in its upper part ; hence the 

 shell is irregularly conical. The upper part is smooth, or only slightly 

 striated, in a transverse direction, agreeable to the successive addition of 

 new matter to the shell. The opening, which is nearly semicircular, 

 possesses about one third of the base. The size of some of these fossils 

 is very considerable. Lamarck observes that the width of the largest spe- 

 cimen is seven centimetres (about two inches Fr. and seven lines.) One 

 of the specimens 'which I possess is hardly more than an inch across its 

 widest part; whilst another, which I purchased from the collection of 

 M. de Calonne, measures in the same direction full three inches and three 

 quarters, and exceeds two inches in height. These gigantic proportions 

 widely distinguish it from any recent shell of this genus. These fossils 

 are from Retheuil and Courtagnon. I am not acquainted with the dis- 

 covery of any shells of this genus among our English fossils. 



Plate VI. Fig. 4, is a curious fossil, being a calcedonic cast of the 

 hollow of a nerite of this species, displayed by the removal of the top of 

 the shell, 



