296 REPORT OF NATIONAL MUSEUM, 1802. 



si t ion 4. The second part finds abundant illustration in tlie great dis- 

 parity of advancement in rank between molluscan and mammalian life 

 during Tertiary time and between molluscan and dinosaurian life dur- 

 ing Mesozoic time. The disparity is seen to be all the greater if only 

 the fresh water mollusca are considered, the slight differential develop- 

 ment of which during successive epochs of geological time is mentioned 

 and referred to in following paragraphs. 



In these essays the term type* is usually employed with reference to 

 the chronological significance of the forms or groups of forms thus des- 

 ignated. Their succession in time was not necessarily coincident with 

 progressive evolution, but they were always the result of differential 

 evolution. Their chronological value to the geologist depends upon the 

 dehniteness of the limits of their time range. Usually their time range 

 was comparatively short, but sometimes they continued their existence 

 through long periods of time. A considerable number of types are 

 specially characteristic of each stage of the geological scale, but any 

 of them were liable to range beyond its limits. 



(5) The rate of differential evolution among the forms constituting certain 

 divisions of the animal and vegetable kingdoms was greater than that among those 

 constituting other divisions; and it was greater for some of the members of a given 

 division under certain conditions than it was for other members of the same divi- 

 sion under other conditions. 



The truth of this proposition may be more clearly shown by exam- 

 ples than by explanation. Some of the most remarkable examples of 

 slight differential evolution during a succession of geological periods 

 being furnished by fresh water and land molluscan faunas, these may 

 first be mentioned. 



fossil remains of numerous fresh water gill-bearing molluscan fau- 

 nas have been found in North American strata belonging to nearly 

 every geological period from the Jurassic to the post Tertiary, inclu- 

 sive, each fauna consisting of members most, and sometimes all, of 

 which belong to genera that are abundantly represented by living 

 species; that is, only a small proportion at most of extinct genera, 

 and no extinct families are known to have been included in any of 

 these faunas. In short, the differential evolution of the North Ameri- 

 can fresh water mollusca during the Mesozoic andCenozoic eras seems 



Because the term "type" is used in theseessays in a special and alsoasomewhat 

 variable sense, it requires explanatory definition. It is herein used to indicate groups 

 of animal or vegetable forms which have certain recognizable but often difficultly 

 describable, characteristics in common. Such groups are sometimes identical with 

 genera, but "hey are sometimes less, and often more comprehensive, even embracing 

 families and. in rare cases, orders. They usually have only an incidental relation 

 to the groups which are recognized as divisions of the systematic classification of 

 animals and plants, but they sometimes coincide with them or constitute accessory 

 features of such classification. Types thus designated are recognizable by general 

 form, peculiar details of essential parts of struct lire, accessory features, or a general 

 summary of peculiarities of structure or biological characteristics. 



