7:>s 



Report of the State Geologist. 



Tullv limestone (Cuboides horizon) of New York. Though the group is 

 well represented in lower Devonian horizons, there is no record of its per- 

 durance beyond the Cuboides horizon, and we notice that in this late represen- 

 tative of Thysanopeltis the marginal spines are minute and hair-like. Within 

 the limitations of this group we find the glabella passing through variations 

 in dimensions quite similar to those which characterize the genus as a w hole. 



Mronteus senesoens is devoid of marginal spines and reproduces the char- 

 acters of the normal middle Devonian type. We look upon this species, 

 therefore, as distinctly progressed in the character of its glabella and pygi- 

 dium and as having escaped entanglement with the early Devonian divergence 

 into Thymnopeltift ; a survival, with appropriate time modifications, of the 

 proper expression of the genus. 



