CERITHIUM. 14a 



different. If the diagnosis of Diartema were somewhat altered, it might be made to 

 include the species here described as raricifera. 



This is another instance where the Aporrhaidse of the Lincolnshire Limestone 

 at Great Ponton show their affinity with Bathonian forms. 



Family— CERITHIIMJ. 



"Shell spiral, elongated, many-whorled,, frequently varicose ; aperture channeled 

 in front, with a less distinct posterior canal; lip generally expanded in the adult." — 

 S. P. Woodward* 



The above diagnosis was no doubt formed so as to include both Nerincea and 

 Aporrhais, both of which are now excluded. Fischer observes that the shell is 

 very variable ; the canal, generally short, becomes rudimentary and even disap- 

 pears completely, as though to show in some sort the impossibility of separating 

 the Siphonostomes and the Holostomes. He observes that the same modifications 

 of the canal are observed in the Melaniidse. I would here remark on the 

 possibility of the more recent Melaniidse having originated from some of the old 

 Jurassic Cerithiidse, or at least from molluscs of that age, which we group with the 

 Cerithiidee. Most freshwater Gasteropoda are probably descended from genera 

 which were once marine. 



In the Inferior Oolite the Cerithiidae are well represented, though some genera 

 are placed here with a query — Brachytrema, already described, is thus classified 

 by many. The following genera are referred to this family. Gerithium, Adanson, 

 Fibula.,, Piette, Geritella, Morris and Lycett, Exelissa, Piette, Gryptaulax, Tate, and 

 Gerithinella, Gemmellaro. Of these Cerithinella, and possibly even Gryptaulax, 

 might almost claim relationship with the Turritellidae. 



Genus — Cerithidm, Adanson, 1757. 



" Shell imperforate, turrited ; whorls numerous, narrow ; the last always shorter 

 than those of the spire ; aperture oblong, semioval ; anterior canal short, oblique, well- 

 marked; lip more or less thickened" — Abridged from Fischer. 



The Jurassic Cerithia cannot be judged by the same strict diagnosis as may be 

 applied to existing species. A considerable group of fossil shells, many of them 

 small, occur in our Liassic and Oolitic strata, to which the generic name 

 Gerithium is applied. Some of these fossils are, perhaps, more like Bittium, where 



