found in the Nile by M. Caillaud. 519 



might be .considered, at least, as of a problematical nature. It 

 might be concluded from this fact, that at a time when animal 

 life and vegetation were so different from what they are at pre- 

 sent, Oysters might live in fresh water and consequently alterna- 

 tions, founded upon the presence of any of these shells, might not 

 be admitted. 



Already several genera, which were thought to be exclusively 

 marine, such as the Mytiliy the Modiolw, and the Corbulce, have 

 been found in fresh water. If to these are added, such genera 

 as are only found in fresh water rivers or lakes, such as Anodon y 

 Unto, and Cyclas which are found every where ; the Cyrena and 

 Jridina which live only in the rivers and lakes of the East and 

 of India; the Hyria and Castalia of America, and the Galathea of 

 Ceylon, we shall have enumerated all the genera of bivalve mol- 

 lusca, not exclusively marine. 



Not a single irregular shell, of a thick, foliated structure, ad- 

 hering to the rocks or to each other, and consequently deprived 

 of locomotion, and varying in form according to the nature of the 

 substance to which they adhere, and by their accumulation pro- 

 ducing true beds (couches), had as yet been discovered in fresh 

 water : such, however, are the Shells which M. Caillaud has 

 found, and the following is the information we have received 

 from this zealous Traveller. " We first meet with them," says 

 he, " after passing the first cataract ; and they do not appear to 

 exist below ; they become very abundant in the province of 

 Rebata, beyond the Peninsula of Meroe. The inhabitants collect 

 them on the banks of the river, to ornament their tombs with 

 them, and they say that they come from the more elevated parts 

 of the Nile, from Sa'ida, where they are eaten." M. Caillaud 

 has found them as far as Fazoql, the most distant country into 

 which he has penetrated upon the Blue River. In Senaar, the 

 inhabitants informed M. Caillaud, that during the summer season, 

 when the river was low, they took them with the animal, but 

 notwithstanding all his endeavours, M. Caillaud could not obtain 

 any living specimens, the river being always too high. They are 

 said to be very common in the Jaboussi, a river which runs into 

 the Blue River, and in all appearance the numerous confluent 



