PALAEONTOLOGY. 27
2. At each great geological epoch there existed not only distinct species, but alsogenera and zoological forms peculiar to it.
3. No transition being observed in the specific forms from one formation toanother, it may be assumed that the succession of organized beings on the surface ofthe earth was not by gradual passage, but by the extinction of existing races, andthe creation of new species at each geological epoch.
4. These conclusions, drawn from the Gasteropoda , confirming in every respectthose which have been obtained from the Cephalopoda , it appears that two dis-tinct sets of Molluscs , the one, the Cephalopoda , the inhabitants of deep seas, andthe other, the Gasteropoda , the inhabitants of the more shallow water of the seacoast, have combined to testify that the earth has been again and again depopulated,and again renewed in its living creatures.
The great laws which have been thus traced in the progressive distribution oforganic bodies, from the consideration of the Cephalopoda and Gasteropoda , are con-firmed by the next great class , the Lamellibranchiata . These are the headless Molluscswhich inhabit bivalve shells. Not possessed of the means of rapid movement likethe Cephalopoda , or of crawling on the surface like the Gasteropoda , they are, for themost part, nearly sedentary in their habits, and consequently are the more liable tobe limited in their extension, as they are the less able to remove themselvesfrom injurious influences. D’Orbigny has described 553 species in the Cretaceous fauna, but we shall only quote from him a few of the genera which either connectgenerically the Cretaceous with the Oolitic period, or which, first appearing in theChalk, are still members of the existing fauna, or which are peculiar to the Chalk.
Opis. This genus, though possessing a general resemblance to the genus Cardium(or Cockle), is very strongly distinguished by its very large and prominent beaks. Itis peculiar to the Oolitic and Cretaceous periods, being most abundant in species inthe former, though it has supplied to the latter six species.
Crassatella — a living genus in hot climates. The inhabitants of sandy shores,they bury themselves vertically in the sand. They commence with the Cretaceousperiod.
Cardita, Cyprina, and Lucina are considered by D’Orbigny to commence with theChalk.
Trigonia. This genus, so strongly marked, and so characteristic of the Ooliticperiod (although one species is assigned by D’Orbigny to an earlier), abounds inthe Cretaceous , producing there 21 species; yields only one to the Tertiary , andthen in the recent period forms part of the extraordinary fauna of Australia, —ahistory which suggests to the mind many interesting speculations.
Myoconcha —peculiar to the Oolitic and Cretaceous periods.
Clavagella. This very remarkable recent genus yields its first species to theChalk.
Legnminaria — a genus which gives very few species to the recent fauna, and gaveits only fossil one to the Chalk.
Fistulana —first appears with the Chalk.
Lavignon (Lutraria of Lamarck )—begins in the Cretaceous period.
Arcopayia (separated from Tellina)—first occurs in the Chalk.
Thetis —peculiar to the Chalk.
Gervilia —common to the Oolitic and Cretaceous periods, and then ceases to exist.
Inoceramus —abundant in species, and ends in the Chalk.
Janira —a genus now separated from Pecten, &c., containing the formerly so-calledP. quinquecostatus, P. quadricostatus, &c., begins in the Cretaceous period, is traceablein the Tertiary , and still exists.