Vol. 58. ] MR. F. R. C. REED ON THE GENUS LICHAS. 59 
6. Norss on the Genus Licwas. By Freperick Richard CowPrr 
Reep, Esq., M.A., F.G.S. (Read November 20th, 1901.) 
I. Inrropucrory Remarks. 
Various attempts have been made ‘to separate the species of the 
genus Lichas into subgeneric groups; but the results arrived at 
have not heen altogether satisfactory, and emphasize the difficulty . 
of deciding as to what are the important structural features which 
should determine the classification. The fragmentary state of the 
material available has considerably increased the difficulty, for 
very few species are known completely ; and in the majority of 
cases we have to be content with the evidence of isolated head-shields 
and pygidia. For this very reason the system of classification 
employed by Barrande’ is not capable of general application, because 
it is based on the characters of the thoracic pleure. The majority 
of European species. would (by Barrande’s system) fall into his 
third group, which contains a heterogeneous mixture of forms 
that are only known by their pygidia or head-shields. Apart 
from the general rarity of the preservation of those very parts 
which Barrande’s classification demands, there is a narrowness 1n 
the system which fails to commend it to a paleontologist who 
attaches importance to the combination of structural characters, 
rather than to the presence of a single feature irrespective of other 
differences. A consequence of Barrande’s method is seen in his 
Group 1, in which there are species associated together that possess 
head-shields showing most fundamental differences. 
The basis of a natural and phylogenetic classification of the 
Trilobita, as opposed to an artificial one, has been found in the 
structural characters of the head-shield; and the principle has been 
shown to be safely applicable in all the minor subdivisions and 
groups, as far down as genera and subgenera. 
The variations in the form and lobation of the glabella in the 
Lichadidee evidently indicate, as Dr. Beecher® has truly remarked, 
‘ differences in the relative development of the appendages and organs of the 
head, and therefore are of considerable morphological importance.’ 
II. GENERIC AND SUBGENERIC SUBDIVISIONS. 
* There has been no general agreement among-paleontologists as to 
the number of the subgenera. Prof. Zittel’® in 1885 gave two torms, 
Lichas and Terataspis, as of generic value in his family Lichade, 
and mentioned Platymetopus, Hoplolichas, and Conolichas as sub- 
genera of Lichas, while the following names are enumerated as 
synonyms of Lichas :—Platynotus, Arges, Metopias, Archinurus | sic], 
1 ‘Syst. Sil. Boh.’ vol. i (1852) p. 595. 
~ Amer. Journ. Sci. ser. 4, vol. iii (1897) p. 197. 
* *Handbuch der Paldontologie’ vol. ii (1885) pp. 623, 624. 
