PEaSISTENCE OF TTPE-STEUCTURE. 207 



certain groups are much longer or shorter lived than others of a 

 very closely related nature. 



Persistence of Type-Structure. — The persistence of certain 

 type-structures is very remarkable. Not only have they in a meas- 

 ure resisted all the modifying influences which Nature has brought 

 to bear upon them during a period of hundreds of thousands or 

 millions of years, but in such a manner as to render a specific sepa- 

 ration of their newest and oldest representatives a matter of con- 

 siderable difficulty. The Lingula of to-day differs but little from 

 the Lingula of the oldest fossiliferous formation with which we 

 are acquainted, the Cambrian, although the interval of time sepa- 

 rating the two has been variously put by geologists and physicists 

 at from one hundred to three hundred millions of years 1 The 

 pearly Nautilus is but little removed from some of its most ancient 

 representatives of the Silurian and Devonian seas, and, indeed, it 

 may be considered doubtful whether some of the existing Foram- 

 inifera are at all different from forms that occur deep down in 

 the Paleozoic series of deposits. The number of generic types that 

 have survived the Paleozoic era to the present day are sufficiently 

 numerous, but these belong as a rule to the lower classes of organ- 

 isms. Among the Brachiopoda we have at least five such genera 

 — Lingula, Discina, Crania, Rhynchonella, and Terebratula — and 

 two of these, Lingula and Discina, date back to the Cambrian 

 period. The Acephala and Gasteropoda furnish an equally large 

 number — Pecten, Mytilus, Nucula, Leda, Pinna, Lima, among the 

 former, and Pleurotomaria, Capulus, Turbo, Natica, Dentalium, 

 and Chiton, among the latter — and it is by no means improbable 

 that many of the older genera, now recognised as distinct by reason 

 of our imperfect knowledge concerning their true relationships, 

 have in reality representatives living in the modern seas. Of the 

 Paleozoic Cephalopoda we have but a single surviving genus. Nau- 

 tilus. Several of the recent genera of entomostracous Crustacea 

 (Estheria, Cypridina, Apus) range back in time to the Devonian or 

 Carboniferous periods, and a more limited number (Bairdia, Cythere) 

 even to the Silurian ; but of the higher decapodous types we meet 

 with no (even doubtful) modern generic representatives until the 

 Carboniferous limestone is reached (Astacus Philippi, a supposed 

 species of crayfish, from the mountain limestone of Ireland). The 

 king-crab (Limulus), not unlikely a descendant from the more an- 



