144 GASTEROPODA OF THE INFERIOR OOLITE. 
the anterior canal is short, indistinct, and scarcely reflexed. M. Cossmann 
observes that out of forty-one species of Cerithia recorded from Bathonian beds 
in France, probably not more than half really present the characters of Cerithium. 
We might almost say the same of many species in our Inferior Oolite. But if we 
were to remove them. from that genus it would be necessary to make a number of 
allied genera in order to receive each species. In the case of forms now existing 
where the shell is well preserved and the animal can be put in evidence, such 
multiplication of genera is justifiable, and is even necessitated by the enforcement 
of the binomial system of nomenclature. To adopt this method where imper- 
fectly preserved shells are alone obtainable is scarcely to be recommended. The 
result is that Cerithium, as a Jurassic genus, is not exactly the same thing as the 
recent genus, especially as now restricted. The Jurassic Cerithia have not the 
anterior canal so well developed or so strongly reflexed, and in some cases the 
canal is very poorly developed indeed, a defect which is aggravated by imperfect 
preservation. 
It thus happens that species have been ranged under “ Chemnitzia’’ which 
ought to be classified under Cerithiwm. This leads to the consideration of a 
question which is best discussed at the present stage, viz. what is to become of 
“ Chemnitzia.” It is scarcely necessary to point out that D’Orbigny’s “ Chem- 
nitzia”? of 1850 and his “ Chemnitzia”’ of 1839 refer to distinct groups of 
shells. The ‘‘ Chemnitzia”’ of 1850 is now very generally referred to Pseudo- 
melania, Pictet and Campiche, a genus well represented in the Inferior Oolite, 
which will be dealt with subsequently. D’Orbigny’s original “ Chemnitzia” 
is now held to be a synonym of Turbonilla, Leach (vide Fischer, p. 789), a name 
applied to a group of shells which are elongate, narrow, and many-whorled, the 
whorls being slightly convex and usually costulated ; the aperture is simple, and 
oval or subquadrangular. This genus is said to extend no farther back than 
the Tertiaries. In the Bathonian of France, Cossmann refers no species to 
* Chemnitzia.” On the other hand, Gemmellaro, and after him Tryon, accept 
Chemnitzia, D’Orb., as a genus of fossil shells, divisible into the sub-genera 
Chemnitzia, Rhabdoconcha, Pseudomelania, Oonia, and Microschiza. The four last 
sub-genera evidently belong to the Pseudomelaniadz, and need not be discussed 
here. There remains, then, Chemnitzia, D’Orb., as restricted, of which one of the 
types is Chemnitzia (Turritella) undulata, Benz. The following is Gemmellaro’s 
diagnosis: ‘‘ Shell elongate, composed of a great number of whorls transversely 
(axially) plicated. Mouth oval, rounded or angular in front, confined posteriorly. 
Columella straight or sometimes curved, and slightly incrusted. Lip thin and 
sharp.” Ido not think that there are any species in our Inferior Oolite which 
come within the above diagnosis. There are one or two cases where Cerithia, of 
the section originally grouped by Deslongchamps under Melania, have the anterior 
