With respect to the relative age of the Monte Hermoso and Punta Alta beds, it is not possible to speak decidedly. A certain degree of similarity in the nature of the strata containing quartz pebbles, and those of the reddish indurated earth; and the short distance between the two localities, would indicate that no long interval had intervened. The beds at Monte Hermoso, certainly were deposited more tranquilly, and probably in a deeper sea; so that even skeletons of animals, no larger than rats, have been perfectly preserved there. In some parts of the surrounding country, obscure traces of a succession of step-formed terraces may be observed; and each of these indicates a period of repose during the elevation of the land, at which time the strata previously existing were worn away, and fresh matter deposited. The Monte Hermoso beds were, perhaps, formed during one such interval, anterior to the accumulation of the shingle bank at Punta Alta. Mr. G. Sowerby, who has been good enough to examine the shells which were found with the remains of the quadrupeds, has given me the following list. 1. _Voluta angulata._ 2. —— _colocynthis._ 3. _Oliva Brasiliensis._ 4. —— Nearly related to _O. patula_, but specimen imperfect. 5. —— Nearly related to _O. oryza_; less nearly to small species now living at Bahia Blanca. 6. —— _Nov. spec._ 7. _Buccinum cochlidium._ 8. —— _globulosum._ 9. —— One or two minute species, perhaps young specimens,—unknown. 10. _Trochus_ _Nov. spec._ (?) same as one now living in the bay. 11. —— _Nov. spec._ (?) nearly related to last; differs in not being granular on the surface. 12. _Assiminia_ (?) Minute species, identical with one living in the bay. 13. _Bulinus nucleus._ 14. _Fissurella_ Probably same as a kind (_nov. spec._ ?) living in the bay. 15. _Crepidula muricata._ 16. —— _Nov. spec._ 17. _Cytheræa_ Closely related to, or identical with _C. purpurascens_. 18. _Modiola_ Same as recent kind (_nov. spec._) living in the bay. 19. _Nucula_ Near to _N. margaritacea_. 20. _Corbula_ Minute species, unknown. 21. _Cardita_ Ditto ditto 22. _Pecten_ _Nov. spec._ (?) very imperfect specimen. 23. _Ostrea_ Oysters of the same size now live in the bay. I may add that a fossil encrusting coralline is the same with one now living in the bay.