600 Report of the State Geologist. 



missible. Thus Fenestella and Polypoea. have been placed in 

 different families, yet both forms very gradually merge into the 

 genus PoLTPORELLA. A full discussion of the different genera will 

 be found in the paper above referred to. 



All the forms have a general resemblance to Fenestella and 

 are separated from it by the number and disposition of the cell 

 apertures, difference in the structure beyond the celluliferous 

 face, mode of growth, etc. 



Fenestella, Miller. 

 Type, Fenestella antiqua^ Miller. 



(Plate 2, figs. 1-17.) 



This genus was first proposed in manuscript by J. S. Miller 

 of Bristol, England, but the first published description of it was 

 given by W. Lonsdale in Murchison's Silurian System, Pt. II, 

 p. 6YT. 1839. 



Diagnosis. A ramose, calcareous bryozoum, ' forming cup 

 shaped or funnel shaped expansions ; branches bifurcating and 

 connected by apparently solid dissepiments ; cell apertures 

 occurring on one side of the branches ; arranged in two parallel 

 rows, which are separated by a carina or row of nodes. Reverse 

 side consisting of a stratum of fibroid, calcareous tissue, which 

 is traversed by numerous minute tubuli, at right angles to the 

 surface. For illustrations of this genus, see Plate 2. 



The species of this genus have been placed in the following 

 groups based upon the character of the carina. 

 Group a, for forms with low, smooth carina. 

 Group /?, for the forms with a range of nodes between the 



cell apertures, or a low, nodose carina. 

 Group r^ for forms with very prominent, equal, thin carinaB. 

 Group (5, for forms having carinao expanded midway of their 

 height, then contracting, the expanded portion 

 having nodes on its margins. 

 Group £, for forms with prominent carinse, summits expanded, 



margins smooth. 

 Group C, for forms with prominent carinse, expanded at the 



summit, margins nodose. 

 Group ^, for forms with prominent, moderately thin carinae, 

 having conspicuous lateral projections at the 

 summit. {Fenestella perjylexa.) 



