282 GEOLOGY. 



and pygidium, or caudal shield. The variations of these divisions 

 and of the other parts of their structure furnish the basis on which 

 genera and species are founded. As the fleshy parts and the delicate 

 understructure are wanting, the descriptions and identifications of 

 species and genera are based solely on these harder parts. This is 

 not exceptional. The determination of most fossil species rests on 

 such imperfect knowledge of the real organism, but the imperfection 

 of knowledge is being gradually reduced by the finding of forms that 

 preserve what is lacking in those previously found. Much of the 

 value of these, and all other fossils, in the correlation of formations 

 in different regions, and in determining the succession of beds when 

 not "superposed, depends on the sharp discrimination of their differ- 

 ences of form and structure, and a critical study of the figures is recom- 

 mended. In a work of this kind it is impracticable to enter upon 

 the special description of fossils; that is the function of paleontology; 

 but some special attention may be given to these first forms, the leaders 

 in the gpeat procession of life, so far as we know it. 



The^-general aspect of the trilobites at once discloses an advanced 

 development. It seems clear that they possessed nearly all the 

 anatomical systems and physiological functions of modern crustaceans 

 of their kind. In their line, they were much nearer the present end of 

 the series than the theoretical beginning. Perhaps the eyes are the 

 best index of their development. These show that the compound 

 system of fixed eyes, composed of many eyelets, was already well 

 developed. In the trilobites of this and succeeding periods, the eyelets 

 range from a score to several thousands. Some of the Cambrian trilo- 

 bites however had no eyes, never having acquired them, while others 

 possessed abortive rudiments of eyes, implying that their ancestors 

 had possessed eyes, but had lost them. The acquisition and abortion 

 of so important an organ seems to imply variation in the conditions of 

 life, but this may mean no more than migration to deep dark waters, 

 or the habit of burrowing in the mud, where eyes became useless. The 

 eyes were often slightly raised on crescentic lobes, with the convex 

 face outwards. In later epochs, these crescents became more and 

 more curved, extending the sweep of vision fore and aft to the trilobite's 

 obvious advantage, and hence this became a mark of progress. In the 

 Cambrian faunas, the segments of the body were rather stiffly joined 

 to one another, but in later epochs they became more flexible and some 



