CLINTON GROUP. 



89 



GASTEROPODA OF THE CLINTON GROUP. 



The Gasteropoda are about equally conspicuous with the Acephala, individuals of a few 

 species occurring in all the eastern localities of the group, while they are more rarely seen on 

 the west of the Genesee river. Since many of these occur only as casts in a ferruginous shaly 

 sandstone, they present no reliable characters, and consequently in this condition are of less 

 interest. Like the Acephala, the Gasteropoda appear to be restricted in their geographical 

 range, and indeed few of the species occur in more than one locality. The same remark there- 

 fore will apply to them, viz : that those species now presented can not be regarded as so com- 

 plete a representation of this class in the group, as we have of the Brachiopoda. 



Referring again to what has been said regarding the variable lilhological character of this 

 group, we shall at once trace this influence upon the fauna. Thus nearly all the Gasteropoda, 

 as well as Acephala, are restricted to that part of the group east of the Genesee river ; while 

 the Brachiopoda peculiar to this portion are few in number, and of far less importance than 

 those which occur in the group from Wayne county westward. The inference, therefore, that 

 the physical conditions of the surface have had much influence upon the organic existences, 

 is sustained in this instance. 



Genus CYCLONEMA {nov. gen.). 



[Gr. KuxXoj, ambitus, and vsjAa, filum, in allusion to the elevated threadlike striae marking the 



surface of several species.] 



Shells turbinate, thin ; spire short, consisting of few volutions which increase rapidly from 

 the apex ; aperture large, rounded anteriorly, and somewhat flattened on the colmnellar side ; 

 umbilicus none ; surface strongly marked by spiral threadlike strice, which are cancellated by 

 finer striae. 



The shells of this genus Include forms which have been referred to Pleurotomaria, Litto- 

 RiNA, &c., but which have no slit or indentation in the outer lip, or band upon the volution. 

 The surface is marked by elevated striae parallel to the direction of the volutions, and the spaces 

 between these are marked by finer striae crossing the others obliquely ; the latter, however, are 

 often obsolete. 



The Pleurotomaria hilix of Conrad*, may be regarded as the type of this genus. The 

 following species have the volutions a little more gibbous than in P. hilix ; but the general 

 characters, such as form of aperture, striae, etc. are similar. 



• See Palfflontology of N. York, Vol. i, pag. 305, pi. 83, fig. 4. 



[ PaljEont-O'LOoy — Vol. ii.] 12 



