242 GEOLOGICAL BIOLOGY. 



everything recordable of the characters of the whole organism. 

 These shells from their imperishable character are preserved 

 in the rocks in great numbers, so that variations are found 

 for comparative study. The particular consideration of these 

 hard parts and the study of the marks upon them, which 

 have determined the classifications of the paleontologist, can- 

 not be overlooked when it is an historical study we make of 

 organisms. 



Having in view the importance of the characters of these 

 hard parts, of parts which can be examined both in living 

 and fossil condition, Zittel has described and classified them 

 according to the characters which they exhibit in their ma- 

 ture condition after their development and whatever of evo- 

 lution has taken place in their history, are complete. The fol- 

 lowing is a translation of Zittel's description of the MoUuscar 



General Character of MoUusca. — For paleontologists, and 

 particularly for geologists, the Mollusca present a peculiar 

 interest; for all these classes, except the Tunicates, furnish 

 numerous fossil remains. Principally the shells of the Bra- 

 chiopods, of the Lamellibranchiates, the Gastropods, and 

 Cephalopods, are so widely distributed in the formations of 

 all the periods of the earth, that one chooses them in prefer- 

 ence as characteristic molluscs (" Leitfossilien "), wherever 

 the attempt is made to determine the age of the different 

 sedimentary formations. It is quite evident that it is only 

 the calcareous shells, their moulds in stone, or their imprints 

 which are at the service of the geologist. But as these fos- 

 sils are ordinarily distinguished by their characteristic form 

 and by their varied ornamentation, as the classification 

 within the several classes is essentially based upon the char- 

 acters of the shells, there is established a special science, 

 Conchology, which the geologist particularly cultivates. 

 Moreover, although the characters presented by the shells 

 are so insignificant, they are often deceptive; as in the case 

 where the animals of quite different organization (Patella, 

 Ancylus) are able to produce shells absolutely similar: so the 

 classification of shells requires, as in the other divisions of 

 the Animal Kingdom, a firm zoological basis, and the deter- 

 mination of species should be made according to zoological 



