270 CRETACEOUS GASTROPODA 
Rissoina and Rissoa. Deshayes, in his last edition of the Paris fossils, made several 
additions to the number of the genera in this family, and gives a general review 
of the same. He much regrets the want of criticism in Schwartz’ paper, in which 
the genera are provisionally adopted according to H. and A. Adams’ determinations 
in their work on the ‘Genera of recent shells.’ 
The latest attempt at a classification of the Rzssozpa was made by W. Stimpson, 
(loc. cit.), who views the family in a somewhat broader sense, than has been 
done by previous writers. He proposes to distinguish six sub-families, each with 
a certain number of genera. Already before the receipt of Stimpson’s paper I 
intended to group the whole family into at least two divisions. The one was to 
have principally included the brackish, freshwater or amphibious genera, the shells 
of which are usually thin and smooth, and the labrum not thickened externally ; 
the other the marine genera with thick, solid shells and, asa rule, with the labrum 
externally thickened. The animals of all the Rzssorpz# are very similarly formed, 
but those living in fresh or brackish waters have generally no appendages on the 
posterior portion of the foot, and the operculigerous lobes are less developed, than 
in the marine forms. The eye-peduncles are generally united with the tentacles, but 
it seems that their length increases, the more the animal is accustomed to an 
amphibious life. Thus some of the species have the eyes placed near the basis of 
the tentacles, others in the middle, and still others at the tips of the same. The 
differences are very gradual, which makes it impossible to regard them as of any 
important generic value. The length of the rostrum also appears to increase in 
some forms, corresponding with their more amphibious habits; and also, while the 
foot itself often lengthens, its disc at the same time becomes smaller. 
Mr. Stimpson’s researches enter into great details relating to the family, and 
we fully agree with most of his propositions. We would only wish to make one 
or two additions and alterations in the genera, as well as in the sub-families, 
and give therefore a very cursory review of the same, referring for more details to 
the paper quoted above. 
a. Sub-family,—BITHINIIN 4, including— 
1. Bithinia, Gray 1821, (Genera I, p. 341). The first known fossil species 
of this genus are from the Wealden and a large number from the tertiary deposits. 
None have as yet been reported from the cretaceous period. 
b. Sub-family—HYDROBIINZ, with the following genera — 
1. Bithinella, Moqu. Tand. 
2. Stenothyra, Bens.; Deshayes in his new edition of the Paris fossils, describes 
a few eocene species of Stenothyra under the name of Bithinia. 
3. Hydrobia, Hart. Most of the known species have been lately catalogued 
by G. von Frauenfeld (Verh. Zool. bot. Gesellsch., Wien, 1864, Vol. XIV, p. 561, 
ete.; and 1865, Vol. XV, p. 525). 
