110 SoLLtas—On the Origin of Freshwater Faunas. 
the date of their earliest known existence appended. 
It will be seen that Neo- 
thauma is a Paludinid, and consequently a modified freshwater genus—not marine. 
Families 
Genera of Mollusca Date of first known to which the Date of first known 
living in Appearance. Genera Appearance of Families. 
Lake Tanganyka. are assigned. 
Tiphobia, L. Eocene or Up. Cret. 
Paramelania, ; Es 9 
(= Pyrgulifera ?) 20 : : 
Melanin: " ’, | Mid. Juray (Inf, 0o,)/- Melaniina, Mid. Jura (Inf. Oolite). 
Melanella, ij Pliocene; een nen 
Paludomusg, . Mid. Cret., . 
Paludina, Up. Cret., . Paludinids, Mid. Jura (Inf. Oolite). 
Unio, Up. Jura, : ae: : 
Plescdont (), tie Unionide, Devonian. 
JBtheria, Recent, ; 
Spatha, . Up. Cret., . F Mutelida, Up. Cret. 
Mutela, . lL. Eocene, . ; 
Corbicula, L. Eocene, . : : ; 
Cyrena, . Lias, . , Cyrenide, Lias. 
Planorbis, Lias, . 3 
| Neothauma, Recent, i Planorbine, . | Lias. 
| Segmentina, Up. Eocene, ; 
Limnea, Up. Jura, a Nb : 
Physa, is : Limneine, Up. Jura. 
Limnotrochus, L. Pliocene, . 3 a thoes ec 
Lithoglyphus, . Hace : Littorinide, . | Silurian. 
Syrnolopsis,* Recent, | Pyramidellide, | Cambrian. 
Lanistes, Recent, Ampullariide, | Up. Cret. 
Tiphobia, which Mr. Smith regards as one of the most marine-looking of the 
whole fauna, is not only a Melanian, and therefore a genuine freshwater genus, 
but includes Paramelania as a sub-genus; and this is closely allied to, if not 
identical with, Pyrgulifera, which is known as a freshwater form so low down as 
the Laramie beds of North America. Limnotrochus, which Smith speaks of as 
exceedingly like a Trochus, both in general form and sculpture, is assigned by 
him to the Littorinide, a family which is already represented by one freshwater 
genus in the lower Pliocene beds of Europe; and since Lithoglyphus occurs 
together with Limnotrochus in the lake, it is quite possible that both have descended 
* This genus may belong to the Melaniine. 
