DISTRIBUTION OF BEACHIOPODA. 257 



The gradational passage between the brachiopod faunas of the 

 Paleozoic and Mesozoic eras is probably more complete than in the 

 case of any other group of the Invertebrata. Indeed, were it not 

 for considerations drawn from other and higher groups of animals, 

 the Triassic deposits would from this testimony alone be more 

 properly relegated to the former than to the latter of the two eras, 

 for with very insignificant limitations Thecidium, Waldheiniia, be- 

 ing exceptions all the specific forms of this period belong, if not 

 to such genera as are absolutely peculiar to the Trias Koninckia, 

 Coenothyris to types which are not only represented in the Paleo- 

 zoic formations, but in most instances are eminently distinctive of 

 them. It must be noted, however, that by far the greater number 

 of the Paleozoic types had already ceased to exist, and such as still 

 linger on, except the few more persistent types, like Lingula, Dis- 

 cina, Crania, and Rhynchonella, rapidly near extermination. The 

 Cretaceo- Jurassic brachiopods constitute, strictly speaking, a single 

 series, the members of which belong mainly to the genus Rhyn- 

 chonella, and to a number of closely inter-related genera of the 

 family Terebratulidae. 



Perhaps the most striking fact taken in connection with the 

 geological distribution of the Brachiopoda is the remarkable varia- 

 tion shown in the adaptation of different groups to their surround- 

 ings. While certain generic types those of the family Obolida?, 

 for example appear to have been incapable of surviving for more 

 than a comparatively brief period of time, dying out with the sud- 

 denness of their introduction, others, again, like the Lingula and 

 Discina, have persisted throughout all time, and with such slight 

 modification of structure as to render it difficult in some instances 

 to determine specific differences between the most ancient and the 

 most modern forms. That the least complex or most primitive 

 forms of brachiopods should exhibit the greatest persistence might 

 have been naturally expected, but it is nevertheless rather singular 

 that the number of persistent types following these earliest pre- 

 cursors of a class should be so very limited in number. This per- 

 sistence appears the more marked, too, when we reflect how very 

 narrowly circumscribed in their vertical range are the majority of 

 the species. Thus, out of the vast number of described forms, 

 comprising possibly not less than five thousand distinct species, 

 only a bare handful pass from one formation to another, and, iu- 



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