159 



life conditions, viz., the high organisation of these curious 

 Molluscs. They constitute an important section of the highest 

 class of the Mollusca, the Cephalopoda, which with the excep- 

 tion of Insects, must be ranked amongst the most complicated 

 and highly organised Invertebrates. Complexity of organisa- 

 tion is accompanied with greater delicacy, if we may so term 

 it, of physical constitution, which being adjusted to numerous 

 external conditions is singularly liable to be overthrown by 

 changes in the environment. Consequently in the case of 

 the Ammonites, these changes of environment indicated by 

 the varied strata of the Lias, joined with the long period 

 during which the rocks were deposited, may have led to 

 more rapid modifications of form as compared with the other 

 classes of Molluscs. 



4. — The Terrestrial Fauna. 



Table V. 

 Census of the Terrestrial Fauna of Cleveland. 



Total 



1.906 



826 



On the annexed table is summarised the chief results 

 of the researches of zoologists on the land fauna of the district, 

 from which may be obtained a fairly comprehensive idea, of 

 the total number of species of all classes of the animal kingdom, 

 to be found in Cleveland or which have visited Cleveland. Al- 

 though the figure there given, 1,900 species, seems a large 



