142 



NORTH AMERICAN INDEX FOSSILS. 



springing from a thin double plate; vestibules expanding from 

 orifices to angular apertures. Ord.-Sil. 



84. D. diffusa (Hall). (Fig. 197.) Siluric. 

 Several stems originate from a common base, forming a shrubby- 

 frond ; stems frequently bifurcate and spread laterally ; apertures 

 quadrangular or subrhomboidal. 



Rochester shale of New York. 



L. Fenestella Lonsdale. 

 Fan or funnel-shaped reticulated expansion of straight or flexu- 

 ous rigid branches, apertures united by non-celluliferous cross-bars 

 or dissepiments at regular intervals ; two rows of apertures on the 

 inside of branches, separated by a plain or tuberculated median 

 keel. Ord.-Carb. 



85. F. elegans Hall. (Fig. 198.) Siluric. 

 Carina subdued, apertures with longer diameter oblique to direc- 

 tion of branches, which are slender and frequently bifurcating, and 

 are united by thin dissepiments; fenestrules on non-celluliferous 

 side oblong-quadrangular, rarely oval. 



Niagara of New York, Canada, etc. 



Fenestella elegans with enlargements. 



86. F. crebripora Hall. Lower Devonic. 



Fenestrules wider than branches which are connected by dis- 

 sepiments, slightly enlarging at contact; keel faint; non-celluli- 

 ferous side finely striated. 



Helderbergian of Eastern New York, etc. 



