OEISKANY FAUNA OF BECRAFT MOUNTAIN' 



87 



Devonic cliaracters 



Positive elements 



Trilolites. This element is, for the purposes of a diagnosis, of the 

 first importance. The trilobitic genera present are Homalonotus, 

 Bronte us, Dalmanites, Phacops, Acidaspis, Lichas, Pro- 

 etus, Cordania and Cyphaspis. 



The single species of Homalonotus (H. vanuxemi) possesses 

 the strongly annulated pygidium characterizing the earlier representatives 

 of the genus (e. g. H. delphinocephalus of the Niagaran, H. 

 major of the Oriskany) and in this respect is in strong contrast to 

 the latest members of the genus, with obsolete pygidial ribs. On the 

 other hand, the species presents the smooth, unlobed glabella of the 

 prevalent H. dekayi of the Hamilton, and therein indicates a pro- 

 gress from the Siluric type of Triraerus to the Devonic Dipleurae. 



Bronteus barrandii is a species with comparatively small 

 pygidium, having a simple, unbifurcated median rib; representing a 

 group prevailing in the lower Devonic faunas of Europe. 



Dalmanites. The typical expression and phyletic normal of this 

 genus (Odontochile) is represented by a series of Siluric and 

 Devonic forms in which the glabellar lobation is without coalescence 

 and the pygidium more or less caudate. Tliis type is foreshadowed 

 in the lower Siluric (D. achates Billings), and the Niagaran species, D. 

 limulurus, D. vigilans and D. verrucosus, express the struc- 

 ture without modification. The outburst of dermal extravagances which 

 characterizes many Devonic trilobitic genera is foreshadowed in this 

 group in the snouted D. bicornis of the Niagaran, a tendency per- 

 petuated and carried to extreme in D. t r i d e n s and D. n a s u t u s of 

 the Helderbergian. A diiferent style of ornament is evinced in D. 

 pleurotopyx of the Helderbergian (a species which continued its 

 existence into the Onondaga limestone). Here the frontal margin 

 becomes crenulated, and this tendency is more highly elaborated 

 in the D. d o 1 p h i and D. d e n t a t u s of the calcareous Oris- 

 kany. This style of ornament associated with simple glabellar 

 lobation disappeared with the disappearance of D. pleuroptyx in 



