34 



NORTH AMERICAN INDEX FOSSILS. 



30. D. whitfieldi Hall. Middle Ordovicic. 

 Like D. pristis, but hydrothecae with spinose prolongations, and 



base with a single spine. 



Normanskill shales (Trenton) of Hudson Valley. 



31. D. dentatus Brongniart. (= D. pristiniformis Hall.) 



Lower Ordovicic. 



Narrow slender stipes with closely crowded, very oblique hydro- 

 thecae. 



Graptolite shales of Point Levis and Hudson Valley. Lower 

 Chazy. Europe. 



XV. Monograptus Geinitz. 



Rhabdosomes with only a single row of hydrothecae, which in 

 the American form are drawn out into bent points, at the apices of 

 which are the openings. Silur.-Dev. 



32. M. clintonensis Hall. (Fig. 55, b.) Middle Siluric. 

 Stipes straight with well-marked virgula. Ends of hydrothecae 



bent over so that apertures open downward. 

 Clinton shales of Rochester,N.Y. 



XVI. Retiolites Barrande. 



Rhabdosome with the periderm attenu- 

 ated and supported on a meshwork of 

 fibers. Hydrothecae in two rows, opening 

 outward. Two virgulae attached to oppo- 

 site sides, in the median plane. Ord.-Sil. 



33. R. venosus Hall. (Fig. 55, a.) 

 Hydrothecae slightly oblique ; apertural margin concave ; peri- 

 derm very tenuous. 



In the Clinton shales of western New York, associated with the 

 preceding. 



Hydrocorallines. 



Stromatoporoidea Nicholson and Murie. 



The Stromatoporoids are extinct organisms practically confined 

 to the Palaeozoic, where they were extremely important reef-builders, 

 much of the limestone of the Siluric and Devonic resulting from 

 the destruction of reefs of these fossils. They are most closely re- 

 lated to the modern Hydrocorallines (Millepora) and to Hydrac- 



Fig. 55. a (left), Retiolites 

 venosus; &, Monograptus 

 clintonensis ( both enlarged ) . 



Mid-Siluric. 



