OEISKANY FAUNA OF BECKAFT MOUNTAIN 87 



Devonic characters 



Positive elements 



Trilobites. 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. v a n u x e m i ) possesses 

 the strongly annulated pygidium characterizing the earlier representatives 

 of the genus (e. g. II. d e 1 ph i n ocep h a 1 us of the Niagaran, H. 

 major of the Oriskany) and in this respect is in strung 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 Triinerus to the Devonic I) i pleurae. 



Br on tens 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 (Odonto chile) 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. This type is foreshadowed 

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

 1 i m u 1 u r u s , D. v i g i 1 a n s 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. nasutus of 

 the Helderbergian. A different 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 



