INTRODUCTION. 



SECTION I. 



Researches on organic bodies of former ages are equally interesting and necessary 

 to the Zoologist and Geologist, although their respective objects in pursuing such studies 

 are different. For whilst the Geologist is generally satisfied with establishing the difference 

 or identity of the species found in the several strata, the Zoologist insists rather on a 

 perfect knowledge of the animal in question, to enable him thereby to determine the modi- 

 fications which the entire animal organization has experienced in the successive periods of 

 the earth's formation. 



These entirely different interests of the observers explain in some measure why the 

 knowledge of extinct animals necessarily remained in a defective and imperfect state so long as 

 no competent Zoologist occupied himself with the subject, and indeed, even a Zoologist who is 

 so qualified can only give satisfactory information if perfectly acquainted ^ath the organization 

 of the living allies of such animals, and that in most minute detail. This indeed is sufficiently 

 proved by Cuvier's great researches in the department of Palfeozoology, and the example 

 of this great man has led modern Geologists who study fossils to the conviction of the neces- 

 sity of profound zoological studies, and has convinced them that an investigation, at least 

 of the higher animals, cannot be instituted without accurate zoological knowledge. The 

 truth of this principle has, however, been less acknowledged with regard to the lower animals, 

 and least of all with respect to the Articulata, because their number and importance in 

 relation to geology is, upon the whole, comparatively slight, whilst their organization also 

 has been particularly studied only by a few Zoologists, and by them only recently. There is 

 no family, however, among the Articulata of a former world which in every respect deserves 

 so much attention as the family of the Tnhhites ; and consequently this tiibe has been the 

 subject of much research, but our acquaintance with their organization is still very defective, 

 either because all the more recent observers, from a consciousness of their imperfect 

 knowledge, did not enter into the study of them in a zoological point of view ; or because, 

 from the deficiency of their zoological studies, they could not, on attempting to do so, disguise 

 their ignorance on those points. And yet it is undeniable that wc may obtain as clear and 

 perfect an acquaintance with the organization of these creatures as of the Mammalia, since 

 the organization of a crustacean being evidently less complex than that of a mammal, 

 a perfect idea may be developed with even greater completeness from the existing fragments 

 of the Trilobites, than was possible in Cuvier's representation of the Vertebrata. 



