732 THE PALEONTOLOGY OF MINNESOTA. 
LObservations on Phacopide, 
OBSERVATIONS ON THE AMERICAN LOWER SILURIAN PHACOPIDA. 
A sufficient number of species of the Phacopidz has now been described from 
the Lower Silurian of the United States and Canada, to render the discrimination 
between the specific forms a matter of some nicety. Of these species, some are yet 
known only from dismembered parts or isolated heads, but of them all the greater 
number conform to the Pterygometopus type of structure, in the high marginal 
termination of the posterior limbs of the facial suture, the transection of the lateral 
expansions of the frontal glabellar lobe by the anterior limbs of the suture, and the 
usually rounded pygidium without caudal spine. 
The more typical or normal of these species are P. intermedius, P. schmidti and 
P. callicephalus. Of the other Phacopide known from these rocks, Dalmanites achates 
Billings, D. bebryx Billings, D. carleyi Meek, D. breviceps Hall, Chasmops troosti Saf- 
ford and Vogdes, and P. eboraceus Clarke, all show transitional characters in one 
direction or another. Thus we have noticed the difficulty of making a specific 
distinction between Pterygometopus intermedius and P. eboraceus, except in so far as 
the latter, by the incipient coalescence of the first and second glabellar lobes along 
the dorsal furrows, manifests an inclination toward Monorachus, a subgeneric group 
differing from Pterygometopus only in the extreme to which this tendency to coales- 
cence is carried. Dalmanites bebryx and Chasmops troosti*® are species of the same 
character. It would on many accounts be convenient to apply to this developmental 
(in a phyletic sense) stadium of the early phacopidean type, the term introduced by 
Schmidt, Monorachus, but such a designation would fall short of its purpose unless 
accompanied by an equivalent term to designate the same phase of development in 
those early Devonian species which follow the appearance of typical Dalmanites, 
namely, such species as those to which the name Chasmops was applied in the 
Paleontology of New York, volume vii, e. g., D. anchiops Green, of the Schoharie 
grit. 
The first appearance in the lower Silurian of this phase of partial coalescence 
of the first and second lobes was simple; its re-appearance in the Devonian was 
complicated with a variety of ornamental modifications, occurring at a period when 
the trilobites ‘generally were garnished with all sorts of dermal extravagances. 
*This species is described (Proc. Acad. Nat. Sci., Phila.; p. 167, fig. 3) as “not in a condition to record the minor details 
of the head.” The figure, however, shows the third and a part of a large second lobe, sufficient to demonstrate that it is not 
a Chasmops; while the known structure of the species in other respects evinces a close approach to D, bebryx Billings. Chas- 
mops troosti, mentioned by Safford in 1869, but first described and figured by Safford and Vogdes in 1889, is from the Trenton 
horizon at and near Murfreesboro, Tennessee. 
