TKI1.0IHTR& 



Mkl 



N iiif Si IKIKUINATV 61 NIK AMERICAN SPECIES 



or Lien \-. 



Probably in no genus of Trilohites are the characters upon which dependence is 

 usually placed fur taxonomy, so variable as in //iV//./*. Hence arises the fact that 

 essays toward subgeneric division of the very considerable number of known species 

 have been of but very restricted utility. 



The lichads were thin-shelled Crustacea, and in the tenuity of the test and its 



read) ;i.l;i|.t;i1ii>ii tn nmdilicat nni- nf t lie int. -nor may In- fi nun I mic rau-c (if tin- \\ide 



variation in the form of lobes and protuberances, the length of grooves and ridges 

 of the surface. In this respect the genus stands in strong contrast to such compact 

 and thick-shelled genera as I'lmcops where the parts of the test have become rigidly 

 condensed and present throughout the existence of the genus a stable resistance 

 to all modifying agencies. 



The subdivisions of the genus Lichas which have been suggested by the eminent 

 investigators, Angelin. Fr. Schmidt. Dames, and Hall, may perhaps be character i/.cd 

 as well adapted to the material which the authors had before them, and to strictly 

 typical specimens, but losing a degree of applicability when a more extended use of 

 them is attempted. Such a criticism is easily made of any classification, and it is 

 sufficiently evident that these authors were alive to the difficulties presented by these 

 multiform species. 



There has been a diversity of opinion as to the best basis of subdivision. The 

 majority of students have, perhaps, made use of the variation in the lobation of the 

 Isabella, as the most conspicuous and essential source of structural difference, and 

 there can be no doubt of the primary importance of such variation in the trilobites 

 generally. Some authors, appreciating the instability of the characters of the head, 

 have had recourse to the differences in the structure of the pygidium; but this is, 

 also, an equally variable, part. It is evident that any satisfactory classification must 

 take into consideration concomitant variations of all the parts, and in this respect, 

 the elaborate work of I'r. F. Schmidt upon the Silurian species of the Bast-Baltic 

 Provinces must be regarded as the nearest appro xi mat ion to a successful classification. 



Marrande, conservative in his treatment of the classification of all the trilobites, 

 recognized no subgeneric division^, and this is 1>\ far the easiest solution of the 

 taxonomic difficulties arising in the group, hut the strnrtur.il. t'aunal and strati- 

 graphical value of modifications of the generic type are thereby left in obscurity. 

 Subgeneric divisions are iuadmi al.le or useless in series of compact acmic forms 



