162 EOCENE MOLLUSCA. 



to my knowledge, been found in the Upper, although by no means rare in America in beds 

 supposed to be of the age of our Coralline Crag. 



As a recent genus it is widely distributed, but principally confined to tropical or sub- 

 tropical regions. 



The triangular species were erected into a genus by C. B. Adams, in 1845, under the 

 name Thetis, but afterwards changed into Gouldia. 



In this genus and in Cardita a great variety of forms will be seen in specimens from 

 the Lower Tertiaries of this country, as also in f^rance. Some of those which I have 

 admitted to the rank of species may, perhaps, be considered by some Palaeontologists as 

 only entitled to the position of varieties. Whatever may be their claim in that respect, they 

 will at least show well-marked distinctions in form and sculpture ; and it is not, I conceive, a 

 matter of much importance, geologically speaking, whether they be called varieties or species, 

 as they will in either case afford a measure for a palseontological comparison of Faunas. 



The line of separation between species and varieties is in our present know^ledge most 

 arbitrary, some authors sweeping into a single species a number of well-marked forms 

 previously regarded as distinct ; while others, or even the same authors, are erecting into 

 specific importance forms not more, and often even less, distinct from each other than those 

 thus swept into a single species. 



The most rational course, as it seems to me, will be to assign specific value to all those 

 forms which in any given deposit maintain constant characters, and do not by transition pass 

 into others occurrwg in the same deposit. If, in the progress of research into existing 

 Natural History, or into Palaeontology, any of these should be found to "^^^^ geographically 

 into each other, the soundness of their original separation would not be materially 

 impaired, because it would be evident that, notwithstanding this geographical transition, 

 the several races possessed that impress of permanency which caused them to maintain 

 their distinct characters while living together in the same area, or under the same 

 conditions ; and what other than this can be said in justification of the se])aration of any 

 nearly allied species ? When a general repugnance between the respective sexes of any 

 two groups of organisms living together becomes established, then, in my opinion, specific 

 division is accomplished ; for we have the existence of that state of things established which 

 must preclude an intermingling of the races, and offer ready objects for the diverging 

 action of external conditions upon living organisms. 



With these views I must, of course, always feel that there is great uncertainty attaching 

 to many of the specific identities which I have made of new forms nearly allied to some 

 previously known one, whenever the specimen upon which such new form is founded is 

 either unique or very rare, but when a suite of specimens wherein the characteristic 

 peculiarities of either form are available for examination then the specific values attached 

 seem to me more reliable. For this reason many of the forms to which I have attached 

 the species-value, or have restricted to varieties, may be found hereafter to be incapable of 

 being sustained in that category. 



