TRILOBITES. 751 
Observations on Lichas.] 
OBSERVATIONS ON THE SUBORDINATE GENERIC RELATIONS OF THE AMERICAN SPECIES 
or LicHAS. 
Probably in no genus of Trilobites are the characters upon which dependence is 
usually placed for taxonomy, so variable as in Lichas. 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 
ready adaptation to modifications of the interior may be found one cause of the wide 
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 Phacops 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 characterized 
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 
glabella, 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 Dr. F. Schmidt upon the Silurian species of the Hast-Baltic 
Provinces must be regarded as the nearest approximation to a successful classification. 
Barrande, conservative in his treatment of the classification of all the trilobites, 
recognized no subgeneric divisions; and this is by far the easiest solution of the 
taxonomic difficulties arising in the group, but the structural, faunal and strati- 
graphical value of modifications of the generic type are thereby left in obscurity. 
Subgeneric divisions are inadmissable or useless in series of compact acmic forms 
