208 GEOLOGICAL DISTRIBUTION. 



cient line of Trilobites, appears for the first time in deposits of Ju- 

 rassic age. Contrary, perhaps, to what might have been expected, 

 following out the law of the persistence of lower over higher or- 

 ganic types, not one of the numerous genera of Paleozoic corals 

 has survived up to the present period ; and what is stUl more sur- 

 prising, even the broad structural type which they embody has, 

 as far as we now know, almost completely disappeared. What par- 

 ticular conditions tended towards their extermination, and to their 

 being supplanted already in the early part of the Mesozoic era by 

 entirely new structural forms, we know not, and probably never 

 shall discover. 



In sharp contrast to the more persistent types of animal life are 

 certain groups whose appearance and disappearance are alike sud- 

 den, and whose whole existence is measured by a very brief period 

 of geological time. Such, for example, are the Rudistse, a family 

 of acephalous moUusks which attains an extraordinary development 

 towards the close of the Mesozoic era, but all of whose members 

 appear to be restricted to the Middle and Upper Cretaceous periods. 

 Indeed, by far the greater number of representatives of this family, 

 constituting the genus (with several sub-genera) Hippurites, are 

 limited exclusively to the deposits of the Chalk and Chalk-marl. 

 An equally remarkable example of limitation in range is furnished 

 by the Graptolitidse, one of the most widely distributed families of 

 invertebrates with which we are acquainted, not a single undoubted 

 representative of which is known either before or after the Silurian 

 epoch. 



It is a singular fact that all of the more ancient terrestrial 

 air-breathing MoUusca that have thus far been discovered belong 

 not only to modern groups, but mainly, also, to modern genera. 

 Leaving out of consideration the more than doubtful Palseor- 

 bis, which by some is considered to be a land-snail, there have 

 been described, all in a,ll, some seven or eight Paleozoic spe- 

 cies, beginning with the Middle or Upper Devonian, all of them 

 from the deposits of the Noj-th American continent. Three of these 

 have been referred to the genus Pupa, one to Zonites, and three 

 others to genera that have been created for them, respectively: 

 Dawsohella, Anthracopupa, and Strophites. The first of these is 

 not unlikely a true Helix, iwhile the last, based upon a single im- 

 perfect specimen, is too ill-defined to permit of its being classed 



