276 CRETACEOUS GASTROPODA 
by Deshayes as P¢. tuba. The inner space of the aperture is not circular, and 
therefore very different from that of Chilocyclus (Cochlearia, olim); it is moreover 
oval, as in the sub-generic type Zippora, Leach, (Rissoa auriscalpium, Linn.,) from 
which the generic differences are not very great. The recent Scalaria bicarinata of 
Sowerby from one of the Philippine Islands* probably belongs to this genus. 
oe. Sub-family,—RISSOININ 2. 
The genera are well characterized not only by the form of the operculum, but 
especially by the form of their aperture, which is anteriorly effuse or truncate; the 
outer lip being peculiarly produced either anteriorly or near the middle. 
1. Rissoina, d’Orbigny, 1840, (H. and A. Adams’ Gen. I, p. 327); vide 
postea p. 279. 
2. Bacula, H. and A. Adams, 1863, (Ann. Mag. Nat. Hist., XI, p. 18); a 
shell resembling a Hulima, but without enamel covering; the whorls are spirally 
striated, inner lip thickened, as in the next genus, but the outer lip is without a 
varix and produced either in the middle or somewhat anteriorly. The classification 
of Bacula in this place is only provisional. 
3. Keilostoma, Deshayes, 1848, vide postea p. 280. 
Thus viewing the large number of generic forms,+ which have been quoted as 
constituting the family zssorp*, we have before us a large series of apparently 
different shells. Those inhabiting the sea have usually a strong, solid shell, and are 
richly ornamented with strize or ribs; the brackish water forms have a thinner shell, 
though often ornamented in a similar manner, while the shell of the fresh-water and 
the amphibious species is generally thin and smooth. All of them are so closely 
connected by gradual transitions, that there can be little doubt as to their forming one 
natural family, and one to which there is hardly any other equal in importance and 
general interest. It is true that subsequent researches,—especially in fossil Con- 
chology,—will undoubtedly bring us numerous additions, but even with our present 
knowledge of the family, we can be certain that there scarcely appear, in any other 
known group of shells, such gradual transitions from the aquatic to the terrestrial 
forms, comparatively so easily traceable as in this one. The long disputed point 
as to the relations of the ProsopraNcHIA and NEUROBRANCHIA (or operculated 
PuLMONATA,) promises to obtain very valuable explanations through the study 
of the Rissorpm. For the species of Zruncatella, Tomichia, Geomelania, Hydro- 
cena, and others, are exactly those which lead us to Pomatias, Helicina, and others, 
the larger number of which are still found in damp places along the sea-coast. 
Within the family Rzssorpz, the natural succession of the sub-families would be about 
as follows : RISSOININA?, RISSOINE, SKENEIN®, HYDROBIIND, BITHINUN®, POMATIOPSINA, 
TRUNCATELLINE. 
* Lately collected by G. Nevill on the south-coast of Ceylon. 
+ We have excluded from this family the genera Mesostoma (vide p. 189) and Litiopa, both of which are 
classed init by Deshayes. Some of the species of the genus Bugesia, Bourg., probably belong to the 
Viviparipz, and Bugesia Bourguignati, Poladilhe, (Nouv. Miscel., Malacoz. I, fasc. Paris, 1866,) is likely to 
be a form of the Mzzanupz, 
