732 THE PALEONTOLOGY OF MINNESOTA. 



I' '!> .1 |-\ .11 Ions mi rii:in>|j|il,". 



OBSERVATIONS ON THE AMERICAN LOWER SILURIAN IMIACOIMD/K. 



A sufficient number of species of the Phacopida: 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 Phacopidae 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 

 (littering 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 

 1'alasontology 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. 



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