In one collection of these shells, there were some valves of the Venus sinuosa, Lam., the same species found with the Mactra on the banks of the Uruguay. South of Buenos Ayres, near Ensenada, there are other beds of the Azara, some of which seem to have been embedded in yellowish, calcareous, semi-crystalline matter; and Sir W. Parish has given me from the banks of the Arroyo del Tristan, situated in this same neighbourhood, at the distance of about a league from the Plata, a specimen of a pale- reddish, calcereo-argillaceous stone (precisely like parts of the Pampean deposit the importance of which fact will be referred to in a succeeding chapter), abounding with shells of an Azara, much worn, but which in general form and appearance closely resemble, and are probably identical with, the A. labiata. Besides these shells, cellular, highly crystalline rock, formed of the casts of small bivalves, is found near Ensenada; and likewise beds of sea-shells, which from their appearance appear to have lain on the surface. Sir W. Parish has given me some of these shells, and M. d’Orbigny pronounces them to be:— 1. Buccinanops globulosum, d’Orbigny. 2. Olivancillaria auricularia, d’Orbigny. 3. Venus flexuosa, Lam. 4. Cytheraea (imperfect). 5. Mactra Isabellei, d’Orbigny. 6. Ostrea pulchella, d’Orbigny. Besides these, Sir W. Parish procured (“Buenos Ayres” etc. by Sir W. Parish page 168.) (as named by Mr. G.B. Sowerby) the following shells:— 7. Voluta colocynthis. 8. Voluta angulata. 9. Buccinum (not spec.?). All these species (with, perhaps, the exception of the last) are recent, and live on the South American coast. These shell-beds extend from one league to six leagues from the Plata, and must lie many feet above its level. I heard, also, of beds of shells on the Somborombon, and on the Rio Salado, at which latter place, as M. d’Orbigny informs me, the Mactra Isabellei and Venus sinuosa are found.