42 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 16, 



mations older than those of Touraine or Bordeaux. Thus, of the four 

 described species, including Mr. Lea's Petaloconchus sculpturatus, 

 three are confined to miocene beds, and the fourth occurs in both 

 miocene and older pliocene formations. Some probability hence arises 

 that the San Domingo beds which contain this fifth species are also 

 to be referred to the middle tertiary epoch. 



In speculating upon the age of these deposits, the first question is, 

 whether all their organic contents belong to the same formation. 

 Upon this point the evidence of Mr. Heniker, though not a practised 

 field-geologist, deserves great weight. He informs us, that though 

 the fossils were collected from five different localities, by far the greater 

 number were procured by himself from one spot, and the remainder 

 from beds apparently in the same stratigraphical position ; so that he 

 entertains little doubt on this head. As some corroboration of this 

 view, I may state, that the matrix in which the fossils were imbedded 

 appeared to me of only two kinds, and different specimens of the same 

 fossil were frequently incrusted with each. 



Assuming therefore this to be true, we have a deposit containing 

 77 species of MoUusca, of which 13 are unquestionably recent, and 

 2 doubtful. Excluding the doubtful, the proportion of 13 to 7d 

 is exactly \7\ per cent.; and should the 2 doubtful hereafter be 

 identified with recent shells, the proportion of 1 5 to 77 is 1 9^ per 

 cent. 



It is obvious that this formation is of quite a different order from 

 the fossil beds in Barbadoes and Antigua, every species in which is now 

 found living in the adjoining seas. Neither have I been able to iden- 

 tify one shell with those from the older tertiary rock in Barbadoes, 

 described by Sir R. Schomburgk, or those from Jamaica in the So- 

 ciety's collection. If we take as our guide the law of the proportion 

 of recent to extinct forms, the ratio of 1 7 or 20 per cent, would refer 

 this formation to the miocene period of Europe : and the facts that all 

 the Foraminifera and MoUusca which have been identified with known 

 fossil forms occur in beds of that high antiquity, and perhaps in one 

 or two cases older, — the presence of a genus of shell of which no 

 species has been found in beds older than the American miocene or 

 newer than the Sub-Apennines, — the occurrence of the teeth of the 

 great Carcharodon, considered by Agassiz as very characteristic of 

 miocene formations, — together with the general resemblance and the 

 close analogy of some of the fossils to those of Touraine and Bor- 

 deaux, — all seem confirmatory of this view. 



It may seem remarkable that there is no greater specific resem- 

 blance between these shells and those from the miocene beds of the 

 United States. But when it is remembered that the most southern 

 miocene deposits of America are in lat. 33°, while those of San Do- 

 mingo are in lat. 19°, a difference equal to that between Rome and 

 Edinburgh, this will scarcely invalidate the conclusion. 



neither of them has plaits on the columella; and as it is impossible to doubt the 

 accuracy of that gentleman's identification of the recent shells with the Sicihan 

 fossils, I cannot but suspect that these Sicihan shells are distinct from their two 

 synonyms in the Touraine, Bordeaux, and Piacenza beds. 



