314 Notice of the " Silurian System of 



and M. Barrande's observations, as we shall soon see, fully 

 confirm this view. It, however, appears, that in the adult 

 trilobites the number of segments in every part of the body is 

 constant for each species. Emmerich's law of the constancy 

 of the number of segments furnished with pleurae (20), on the 

 other hand, is not established. So also the supposed law that 

 the number of segments in the thorax of each genus was con- 

 stant, has not stood the test of M. Barrande's wide expe- 

 rience, though, singularly, no exceptions have yet been ob- 

 served in the genus Phacops, from which it was first deduced. 

 Still less have his observations confirmed the supposed law 

 of the constancy of the number of segments in the whole 

 body of each genus or family of trilobites, the abdomen gaining 

 in number as the thorax lost, and the reverse. As general 

 results, M. Barrande finds the number of segments varying — 

 in the head, from species in which no segmentation appears, 

 to 6 segments in Paradoxus spinosus and others — in the 

 thorax, from 2 in Agnostus to 26 in Harpes ungula ; in the 

 Pygidium, from 2 in Sao hirsuta, and many others, to 28 in 

 Amphion multisegmentatus ; and in the whole body from 11 

 to 38, or in the last-named species perhaps to 52. In the 

 same genus the number of segments in the whole body range 

 from 24 in Dalmanites solitaria to 38 in D. auriculata. 



The power of rolling themselves up was long considered 

 as highly characteristic of certain trilobites, and even made 

 the basis of some systems of classification. M. Barrande 

 gives many highly interesting observations on this faculty, 

 for which we must refer to his work. Of the forty-five 

 species he studied, twenty-seven have been ascertained to 

 roll themselves up, — in eighteen it has not been ascertained ; 

 but, except Ellipsocephalus, Ogygia, and Paradoxides, only 

 few and fragmentary specimens have as yet been found of 

 these species. On the whole M. Barrande concludes that 

 this power was common to the tribe, and therefore not 

 characteristic of particular genera or species. 



M. Barrande next notices the forms and characters of the 

 pygidium, but we pass on to the section in which he treats 

 of " the feet and organs of the trilobites." In regard to the 

 former, his observations only confirm the fact that these or- 



