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 Gowldia. 
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 France. Some of those which I have 
admitted to the rank of species may, perhaps, be considered by some Paleontologists 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 palzeontological comparison of Faunas. 
The line of separation between species and varieties is im our present knowledge 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 occurring in the same deposit. Tf, in the progress of research into existing 
Natural History, or into Palwontology, any of these should be found to pass geographically 
into each other, the soundness of their origmal 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 separation of any 
nearly allied species? When a general repugnance between the respective sexes of any 
two groups of organisms living together becomes established, then, m 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. 
