TRILOBITES. 753 
Observations on Lichas. | 
true Lichas. A difficulty, more apparent than real, in assigning the species of 
lichads to their natural groups, arises from the frequent recurrence of extravagant 
styles of ornamentation in groups having a totally different character of cephalic 
lobation. Thus in Metopias, Platymetopus and Hoplolichas may occur a long, club-like 
extension of the frontal lobe; Platymetopus and Conolichas may both have subconical 
frontal lobes, concave on their posterior slope; Platymetopus and Hoplolichas may 
have the frontal lobe garnished with erect tubercle-spines. These are instances of 
morphic equivalence in diverse groups, which cannot be accorded a high significance 
in the association of the species. 
We may suggest the following subgeneric division of the North American 
species of Lichas: 
ARGEs. Goldfuss -—Middle glabellar furrows obsolete, posterior furrows more or less indistinct; 
. outer lateral margins of glabellar lobes convex; third lobes obsolete. Pygidium with 2—3 
annulations on the axis, and narrow, round spines on the margin. A longitudinal ridge extends 
from the extremity of the axis tothe margin. Elevated portions of the body often echinate. 
A, wesenbergensis Schmidt, var. paulianus Clarke. Galena. 
A. phlyctenoides Green. Niagara. 
A. consanguineus Clarke. Lower Helderberg. 
A. contusus Hall. Upper Helderberg. 
L. (Conolichas) hispidus Hall. Upper Helderberg. 
L. (Conolichas) eriopis Hall. Upper Helderberg. 
The last two seem to be more at home here than in the subgenus Conolichas, on account of substan- 
tial differences from the latter in glabellar lobation and form of the pygidium.. 
PLATYNOTUS, Conrad.—Glabella depressed; middle furrows obsolete; frontal lobe depressed at its 
narrowest point; third lobes obscure, but present. Pygidium with three pairs of broad, acute, 
flat spines; axis short with 1—2 annulations. 
L. harrisit Miller. Hudson River. 
L. boliont Bigsby. Niagara. . 
?L. nereus Hall. Niagara. 
L. breviceps Hall. Niagara, 
PLATYMETOPUS, (Angelin) Schmidt.—Lateral glabellar furrows open directly into the occipital 
furrow; no third lobes; all lobes depressed-convex, all furrows narrow; dorsal furrows con- 
cave inward. Pygidium with two pairs of lateral spines and a bluntly bispinous caudal 
termination. 
L. jukesi Billings. Quebec. 
L. minganensis Billings. Chazy. 
L. trentonensis Hall. Trenton. 
L. cucullus Meek and Worthen. Trenton, Galena. 
L. robbinsi Ulrich. Galena. 
L, bicornis Ulrich. Galena. 
CERATOLICHAS, Hall and Clarke.—Frontal lobe elevated, lateral lobes large and highly convex, 
all spiniferous; third lobes absent. Occipital ring broad, with double axial spine. Dorsal 
furrows convex. Ocular node very small. Border broad. 
C. dracon Hall. Upper Helderberg. 
C. gryps Hall. Upper Helderberg. 
TERATASPIS, Hall.—Frontal lobe ovoid, constricted below; dorsal and lateral furrows very broad; 
lateral lobes not sharply defined, highly elevated, directed posteriorly, spinous. Occipital ring 
very broad centrally, with baculate processes. Pygidium with four pairs of long, spiniferous 
caudal processes. 
T. grandis Hall. Upper Helderberg. 
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