34 LOWER AND UPPER SILURIAN FORMATIONS : 



parent a medium at that time as it is at the present day, and 

 that therefore no material permanent alteration can have 

 resulted in either during the thousands of years which have 

 elapsed since the creation of the animal world on this earth." 1 

 The trilobites stand low in the Crustacea, nor are there any 

 appearances of higher animals of that order (such as crabs, 

 lobsters, &c.,) having yet been in existence. 



Of univalve mollusks, which, generally speaking, rank above 

 the bivalves, there are abundant remains. The first and 

 humblest order, Pteropoda, most of which are naked, and 

 therefore incapable of preservation in a fossil state, appear 

 only in a few slight conical shells, indicating an animal allied 

 to the genus criseis, still common in the Mediterranean. Of 



FIG. 10. 



Existing forms of Pteropods, 

 A, Hyalcea; B, Criseis; C, Clio. 



the next, Gasteropoda, there are many fossil species. There 

 are also representatives of the last class, Cephalopoda, amongst 

 which are now found some of the highest of the invertebrate 



1 Emmerich on the Morphology of the Trilobites, Taylor's Sc. Mem., 

 Aug. 1845. 



