white.] INTRODUCTORY REMARKS. 13 



The same might also be said of that portion of the continent which 

 borders upon the Pacific Ocean, and for the same periods, but for the 

 very few Unione forms which the Cretaceous deposits have furnished 

 there, and which have special interest in connection with other fossil 

 Unionida? mentioned in this article. Therefore, almost the whole of our 

 present knowledge of the character of the non-marine molluscan types 

 which existed during the whole of Mesozoic and Tertiary time and of 

 the order of their succession has been derived from discoveries of their 

 remains which have been made in the interior region of the western half 

 of the continent, mainly in connection with the surveys that have been 

 prosecuted under the auspices of the government. Moreover, the dis- 

 coveries that have been made in North America up to the present time 

 give us very little information of any niolluscan fauna, except the marine, 

 for the Miocene epoch, and still less for the Pliocene. Therefore this 

 review of the non-marine molluscan faunae of the continent, although 

 it is intended as a synopsis of all the species that are at present known, 

 is something like a chapter, or parts of chapters, taken at random from 

 a book; but these selections are of such a character as to give us a very 

 good indication of what the whole book, figuratively speaking, must be. 

 This indication is all the more clear because of the fact that while every 

 species that is discussed in this article, from whatever formation it 

 comes, is regarded as extinct, the great majority of the genera, and 

 even the sections or subdivisions of the genera, are precisely the same 

 as those which we find represented by living forms. In the case of many 

 of the fossil forms, so clearly are these familiar generic and subordi- 

 nate types expressed, that the fossil species are often found to resemble 

 those now living so closely as to require careful scrutiny to discover 

 wherein they differ. This persistence through long periods of geologi- 

 cal time, of even the simpler types of non-marine mollusks, after they 

 were once estabbshed, is a remarkable and interesting fact. Individ- 

 uals, generations, and species died, as the epochs succeeded each other, 

 but the types* have remained to this day. 



*The word "type," as used by different authors, has often necessarily a somewhat 

 indefinite meaning ; hut as used in this article it may be defined as an ideal repre- 

 sentation of the essential characteristics of a group of species, usually ajiplied to a 

 group which may embrace a genus, or only a subordinate division of a genus. In the 

 latter case, I use the designation subordinate type. I do not use the term type in any 

 case as interchangeable with any of the names that are used in systematic classifica- 

 tion, such as species, genus, family, &c. ; but sometimes it may be equivalent in scope 

 with any of them ; as, for example, when only a single species of a subgenus, genus, 

 or family is known. 



Thus, although types may have no material existence in one sense, they are found 

 to have been more persistent in time or duration than specific forms; for we find that 

 many of the types, as above defined, which now exist among living mollusca also 

 existed in various geological epochs as far back as Mesozoic, or even earlier, time; 

 but every known fossil species in which those types have been, expressed have suc- 

 cessively become extinct. 



