Bithynia transsilvanica is an eastern European freshwater snail. This species was previously known as Bithynia troschelii.
It is very similar to Bithynia leachii. But Bithynia transsilvanica is larger and it can reach up to 11 mm in shell height. The first three photos are of the same specimen and the width of the shell is 8 mm and the height of the shell is 10 mm.
Another photo is of a smaller Bithynia transsilvanica snail. The width of the shell is 7 mm and the height of the shell is 9 mm. There is seen its operculum, that is slightly pointed. It is an important identification feature:
Beran L. & Horsák M. (2009) “Distribution of Bithynia leachii (Sheppard, 1823) and Bithynia troschelii (Paasch, 1842) (Gastropoda: Bithyniidae) in the Czech Republic“. Malacologica Bohemoslovaca 8: 19-23.
Horsák M., Juřičková L. & Picka J. (2013) Měkkýši České a Slovenské republiky. Molluscs of the Czech and Slovak Republics. Kabourek, Zlín, 264 pp. (in Czech and English). pages 29 and 43.