REPTILIAN AGE. JURASSIC PERIOD. 109 



Operculum thin, testaceous, with a corneous margin; outer surface smooth; pro- 

 vided with two apophyses ; the upper shorter, sometimes dilated and crested, the 

 lateral arcuate. 



The Neritellas mainly inhabit fresh water, but they are often found in brackish, 

 and sometimes even in salt-water. Others are amphibious, and crawl out upon the 

 roots and tnmks of trees along the margins of streams, ponds, and other bodies of 

 water. The genus embraces a considerable number of species which are widely 

 distributed, almost exclusively in tropical and torrid regions. 



Conchologists differ in regard to the limits of this group, some including in it, as 

 sections or subgenera, Dostia, Alina, Neripteron, and Clithon ; while others regard 

 not only these, but some of the others we have here included in the list of syno- 

 nyms, as so many distinct genera. 



The typical Neritellas differ from the closely allied groups Dostia, Alina, and 

 Neripterori, in being more symmetrical, less depressed shells, with a more developed 

 and less eccentric spire, as well as a greatly less expanded aperture, and a less 

 developed lip. 



From Clithon, with which they agree more nearly in form, they differ in never 

 being spinous, and always without a tooth near the upper part of the columella. 

 From the genus Nerita these shells may be distinguished by their much thinner 

 and smoother shells, and smooth or less strongly dentate columella, as well as by 

 the exclusively marine habits of the former. 



It is difficult to arrive at a satisfactory conclusion in regard to the geological 

 range of this genus until the affinities of a number of fossil species have been more 

 accurately determined. It seems, however, to be an older type than Nerita ; indeed, 

 as already intimated, even some Carboniferous forms usually placed in the genus 

 Naticopsis, are very closely similar to Neritella, though doubtless generically distinct. 

 Some of the so-called Neritas from European Jurassic beds probably belong to this 

 genus, though most of them present differences that place them either in the genus 

 Neridomus, or apparently in allied, but undefined genera. If we are right in refer- 

 ring the bed from which the following described species was obtained, to the horizon 

 of the Jura, it would establish the existence of the genus during that epoch beyond 

 doubt, since it is a typical Neritella. 



A few of the Cretaceous Neritas, so-called, seem also to belong here, and the 

 genus was well represented during the tertiary epoch. It attains its maximum 

 development, however, at the present time. 



I¥eritella Mebrascensis. 



Neritella Nebrascensis, Meek & Hayden, Proceed. Acad. Nat. Sci. Phila. Dec. 1861, 444. 



Neritella Nebrascensis. Neritella Nebrascensis. 



Enlargement of bands of color. 



Shell small, obliquely rhombic-oval ; volutions three to three and a half, convex, increasing rapidly in size, the 

 last one composing more than nine-tenths of the entire shell ; suture well defined. Aperture broad ovate. Colu- 



