318 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



almost entirely. For a loug time they were believed to be extinct, 

 To-day, on the contrary, it has been proved that the sea of the Antilles 

 contains representatives of this family. These are the Pentacrinus caput 

 medusae and the Holopus rangii of Martinique. (D'Archiac, op. cit., t. ii., 

 p. 222.) 



Australia presents us with a similar phenomenon. This continent 

 not only possesses animals and plants which recall by their organi- 

 zation the fauna and flora of the Jurassic strata ; but its shores still harbor 

 mollusks which are found elsewhere only in the Lower Tertiary formations, 

 and even in the more ancient strata. Subsequent to the Crinoids appear 

 other echinoderms, particularly the free star-fishes, and the echinoids. 

 The first are found in the Triassic formation, the last in the Jurassic. 

 Several genera are still recent, such as Cidaris, Echinus, Clypeastcr, 

 Ophiura, and Spatangus ; others have had relatively only a very short 

 existence, as NucUolithes, Galeritcs, and Ananchytes. The number of 

 their fossil species is over a thousand. 



The Braclilopods form the largest part of the fossil remains; they 

 therefore render important assistance in the classification of the sedi- 

 mentary depesits, and the determination of the relative age of these 

 rocks. On account, however, of the especial knowledge required of 

 malacology and of fossil remains, such means are of less general use. 



In the most ancient formations predominate the genera Spirifer, Ortis, 

 and Productus, which hardly attain the Triassic system, while the genus 

 Terebrutula, which appears simultaneously with the preceding in the 

 micaschist of the Silurian period, passed through all the following epochs, 

 and is also found in our seas. The other bivalve shells, especially, com- 

 mence to increase in number with the Triassic formation ; in more 

 ancient strata they are relatively quite rare. In the shell limestone 

 abound the genera Avicula, Myophoria, Plagiostoma, Pecten ; the genera 

 Gryphcea, Lyriodon, Ostrca, Pholadomya, in the Jurassic formation. The 

 Chalk contains the genera Exogyra, Inoceramus, Lima, Pinna, and Spondy- 

 lus, while the molasse (a soft Tertiary sandstone), and the deposits which 

 follow, contain the genera Area, Dreissena, Panopea, Crassatella, SoJen, 

 and Pectunculus.* 



The fossil Gasteropods, of which the number of species is very great, 

 are especially frequent in the molasse, but they are found equally in the 

 more ancient deposits ; the shell itself is very seldom found; only its 

 imprint, or the mold of the interior, which renders the determination 

 of genera and species very difficult. 



The Ccphalopods are of great importance in the history of the appear- 

 ance of organized beings, and the part they played in past ages far 

 exceeded that of the present. They form series which distinguish 



* Von Buch observed that the thickest portion of the shell does not occur in the same 

 part with the Braclilopods of all ages. According as the shell belongs to the most 

 ancient sedimentary rocks, to the mean or to the recent formations, the maximum of 

 thickness is found near the ligature, in the middle, or toward the edge of the shell. 



